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In time of rapid change, few things change more rapidly than language itself. Newly discovered phenomena, technologies that until lately have not even existed--each of these demands marked growth in the little conceptual handles that we call words, and that we use as tools to communicate and to record.

Sometimes this growth takes the form of new meanings added precariously to the top of loads carried by existing words, thus setting traps of incomprehension for the unwary. Sometimes the expansion takes the form of the creation, and spreading acceptance, of either freshly minted words or words previously recognized only by small coteries of specialists. The growth of language is a process that races past generally unperceived by most of us. Yet today surely millions of people must have at least a nodding acquaintance with words and terms that, a few years ago, would have quite baffled them in their current contexts, e.g. ablation, staging, midcourse correction, and hold, to cite only a few. At times we are afforded a momentary glimpse of the phenomenon of language change: today we used the word astronaut with casual ease, but only a few years ago as it first crept into the language is sounded bizarre and even pretentious to many ears.

This volume attempts to define the meanings of selected terms in used in areas of activity of the National Aeronautics and Space Administration. The effort has been undertaken because exactness and precision are naturally most lacking in times of rapid language growth; and yet for scientists and engineers precision of meaning can have exceptional, even crucial, importance, as indeed it also can for contracting officers and management in general. But in one sense this book is almost certain to be imperfect. No set of definitions frozen in type can hope to capture the continuing change of living language. Readers of this first edition are cordially invited to submit suggested changes, corrections, refinements of definition, and exclusion to NASA (Headquarters Code ATS) with the thought that subsequent editions can better reflect the actual courses followed by the process of language growth.

Melvin S. Day, Director

Scientific and Technical Information Division


The Dictionary of Technical Terms for Aerospace Use is intended to ease the problem of communication among the various disciplines concerned by providing authoritative definitions of terms commonly used in the literature of space exploration. It is designed for use by a person with a scientific or engineering education who is writing or reading in a field outside his own specialty. It is expected that a person working in his own specialty will use the established references of that specialty which will usually define terms with more rigor and more detail than is possible in this Dictionary.

The definitions are intended to be accurate with respect to their technical content and to reflect the usage of experts in the various fields. The divergence in usage in different fields or within a given field sometimes requires that multiple definitions, which may be somewhat at variance with each other, be ascribed to a single term.

Each definition is intended to be as clear as possible to the non-expert, but accuracy has not been compromised for the sake of readability. Mathematics has been used where necessary to avoid ambiguity.


Work on the Dictionary was started in 1960 under the direction of B.A. Mulcahy, then Director of Technical Information of NASA, who had the responsibility of providing editorial standards for NASA technical publications. This dictionary was planned to supplement the editorial standards issued by NASA Headquarters and the NASA Centers.

It was decided that this Dictionary would:

Coverage: a) Cover both those fields concerned with the space environment, e.g. geophysics and astronomy, and those concerned with space operations, e.g., rocketry and telemetry. Slang would not be included.

b) Include selected terms from the basic sciences and mathematics which were used frequently in the definitions.

Type of Definitions c) Whenever possible use an operational definition (one which defines a concept in terms of actual operations by which the defined quantity can be measured, rather than in terms of properties).

Source of Definitions d) Base definition of definitions already accepted as authoritative by a Government agency, a scientific or technical society, or a national or international standards organization. These definitions would not be accepted uncritically and would be modified when necessary to meet the need of the Dictionary.

This plan has, in general, been followed although limitations on time and personnel in the collecting and editing phases caused the coverage to be somewhat narrower than originally intended. For example, the editor recognizes that the fields of biotechnology and engineering psychology have relatively limited coverage in this edition.


Word List. The first step in the preparation of the Dictionary was the selection of terms to be defined. In the selection process, the editor searched NASA publications, Congressional reports, publications of other Government agencies, existing technical dictionaries and glossaries, authoritative books in the disciplines contributing to space research, current reports on space-oriented research, and engineering journals. Simultaneously, authoritative definitions for the terms and citations showing usage were collected. The product of the search was list of some 8,000 terms. This list was circulated among the NASA technical staff for comment and for recommendations on additions and deletions.

Revised List. Guided by the comments of the reviewers a revised word list was prepared. The number of terms was reduced to 6,000.

Definitions. The definitions compiled during the term selection process were evaluated and were rewritten where necessary to provide a uniform presentation. Where authoritative definitions could not be found, definitions were written based on usage by authorities in the fields concerned.

Review Draft. All definitions were assembled in page format and photographed to provided the plates for the printing of the Review Draft of the Dictionary. In November of 1961, 200 copies of the Review Draft were distributed to members of the NASA technical staff and to other authorities for review and comment. The reviewers provided specific comments, which required changes in more than half of the definitions, and general comments, which guided the reediting of all definitions. The necessary changes were made, and copy was delivered to the printer in July 1962.

Revision. The unavoidable delays inherent in the printing and proofreading processes have been utilized to gather new data that have been used in revising and updating each proof as it was read. It is the editor's opinion, based on his work with standardization activities of several technical societies, that the information is valid as of mid-1964.

Aeronautical Dictionary. This Dictionary owes a great debt to the work of Frank Davis Adams, editor of the NASA Aeronautical Dictionary (now out of print). Many definitions were taken directly from the Aeronautical Dictionary and from source materials collected by Dr. Adams.

Air Force Dictionary. An equal debt is owed Woodford Agee Heflin, editor of the Air Force Dictionary and the Aerospace Glossary. Materials from both publications were used in this Dictionary.

It should be noted that both the Air Force Dictionary and the NASA Aeronautical Dictionary have done much to stabilize the language of aeronautics. Both are widely used by Government agencies and outside organizations, and both have been used as sources for other dictionaries. The NASA Aeronautical Dictionary has also been used as the basis for the AGARD Aeronautical Multilingual Dictionary Supplement, which translates most of the terms for the Aeronautical Dictionary into six languages.

Government Publication. In addition to the NASA Aeronautical Dictionary and the Air Force Dictionary, many other dictionaries and glossaries published by agencies of the U.S. Government have been used as sources for definitions. Most used were the Navigation Dictionary (U.S.N. Hydrographic Office Publication No. 220), the Glossary of Arctic and Subarctic Terms (U.S. Air Force), and the Definitions of Terms Used in Geodetic and Other Surveys (U.S. Coast and Geodetic Survey Special Publication No. 242). Publications of the Inter-Range Instrumentation Group (IRIG) also provided many definitions.

Society Publications. Publications of technical and scientific societies were another important source. The most used of these was the Glossary of Meteorology of the American Meteorological Society. This comprehensive work, sponsored by several agencies of the U.S. Government, served as source for many terms in physics and mathematics as well as in meteorology.

Standards. Many of the definitions in the Dictionary are based on definitions included in the published standards of technical societies and standards organizations.

The principal sources of this type were the publications of the American Standards Association, The Institute of Electrical and Electronics Engineers, and the American Vacuum Society. However, the definitions in this Dictionary should not be used as standards. They are not in the same form as the sources; most are abridged; the terms used in the definition may not be defined in this Dictionary precisely as they are defined in the applicable standard; and, most important they are not referenceable to a specific, dated publication of a standards issuing organization. Users of this Dictionary are urged to turn to published standards in the particular field concerned for the rigorous definitions needed for writing such technical materials as specifications or test method descriptions.

Translation. The International Academy of Astronautics Astronautical Multilingual Dictionary (in press) contains translations of most of the terms in this Dictionary in six languages--Russian, German, French, Italian, Spanish, and Czech. The Dictionary of Technical Terms for Aerospace Use was the source of almost nine tenths of the basic word list of the Multilingual Dictionary.

Bibliography. The principal sources consulted in the compilation of this Dictionary are listed in the Bibliography, at the end of the volume.


The manner of presentation emphasizes the technical content of the definitions and omits information on grammar usually given in general dictionaries. It is hoped that the definitions and the examples will identify the parts of speech and indicate whether a verb is transitive or intransitive. If these points are not clear, it may be assumed that usage is not consistent. Terms are entered in the singular without mention of the plural unless only the plural is used or unless the spelling of the plural is radically different from the singular, e.g. apsides. Plural of apsis. Variant spellings are entered in normal alphabetical order, e.g., apse = apsis. Abbreviations and acronyms are entered in normal alphabetical order.

The order of presentation is (a) the term, (b) symbols or abbreviations, (c) etymology or derivation, (d) definitions and examples, (e) synonyms and cross references, and (f) explanatory note in small type, as

Mach number (symbols M, NMA). (Pronounced mock, after Ernst Mach, 1838-1916, Austrian scientist.) A number expressing the ratio of the speed of a body or of a point on a body with respect to the surrounding air or other fluid, or the speed of a flow, to the speed of sound in the medium; the speed represented by this number. See Cauchy number.

If the Mach number is less than 1, the flow is called subsonic and local disturbances can propagate ahead of the flow. If the Mach number is greater than 1, the flow is called supersonic and disturbances cannot propagate ahead of the flow with the result that shock waves form. Some authorities use mach number but engineering practice is to use a capital M in all words and combinations employing Mach.

Mach wave 1. A shock wave theoretically occurring along a common line of intersection of all the pressure disturbances emanating from an infinitesimally small particle moving at supersonic speed through a fluid medium, with such a wave considered to exert no changes in the condition of the fluid passing through it. The concept of the Mach wave is used in defining and studying the realm of certain disturbances in a supersonic field of flow. 2. A very weak shock wave appearing, e.g., at the nose of a very sharp body, where the fluid undergoes no substantial change in direction.

Typography. Bold face is used for entries, the numeral and letters before definitions, for abbreviations immediately following an entry, and for cross references. Roman is used for the body of the definitions and for notes (in smaller type). Italic is used in lieu of quotation marks to indicate that a term rather than a concept is being discussed. Italic bold face is used for symbols immediately following an entry.

Abbreviations and Symbols. The abbreviations and symbols included are those currently used in NASA publications and those recommended for national and international use by technical societies and standards organizations. For consistency, all abbreviations are printed without points between letters unless omission of points might cause misunderstanding. Omission of a particular symbol or abbreviation does not indicate NASA disapproval of the use of that symbol or abbreviation.

Definitions. The definitions of different senses for a given term are separated by numerals or letters. The letters are used as subheadings under a numbered heading or are used to indicate a close relationship between the different meanings.

Cross References. All terms printed in bold face are defined in the Dictionary. Cross references are indicated by the use of bold face both in the definition and in specific instructions following the definition, such as "see" and "compare". The cross references following the definition should be used for full understanding of the term defined. The cross references within the definition are used to indicate key terms of the definition which are also defined in the Dictionary.

Synonyms. Synonyms for terms defined are entered in normal alphabetical order followed by the symbol, = and a cross reference to the definition; as circle of declination = hour circle. The inclusion of a definition for only one of a group of synonyms does not mean that the used of another synonym is incorrect. It does mean that the evidence examined during the preparation of the Dictionary indicated that the term defined was the most commonly used.

Small-Type Notes. A note in smaller type, following the definition, is used to give additional information about the term defined. The information in a note is not considered essential to understanding of the Definition.

Mathematical Notation. In general, mathematical notations are defined as they occur. Because of the many sources for the definitions, a completely consistent system of notation could not be used throughout the Dictionary.


More than two hundred persons, most of them authorities in specific technical fields, assisted the editor in the selection of terms and in insuring the technical accuracy of definitions. It would have been impossible to prepare a useful dictionary without their assistance. Space does not permit listing the names of all who assisted, and it would be unfair to list only a few. However, specific credit must be given to Mrs. Dorothy Osolinsky who edited all copy for the Review Draft.

William H. Allen, Editor

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