Wordnet 3.0
NOUN (1)
1. 
 a string of characters beginning with a period and followed by one or more letters; 
 the optional second part of a PC computer filename; 
- Example: "most applications provide extensions for the files they create"- Example: "most BASIC files use the filename extension .BAS"[syn: extension, filename extension, file name extension]
WordNet (r) 3.0 (2006):
file name extension
    n 1: a string of characters beginning with a period and followed
         by one or more letters; the optional second part of a PC
         computer filename; "most applications provide extensions
         for the files they create"; "most BASIC files use the
         filename extension .BAS" [syn: extension, filename
         extension, file name extension]