An Etymological Dictionary of Astronomy and Astrophysics

1. An act or instance of extending; the state of being extended.
   

2. Physics: That property of a body by which it occupies space.
   

3. Math.: A field F is said to be an extension of a field K if K is a subfield of F.
   

4. Logic: The sum of all individuals or objects to which the comprehension of a term is applied. For example, the extension of the term “dog” is the set of all (past, present and future) dogs in the world.

See also: Verbal noun of → extend.