Open English Wordnet
Nouns
(n)extension, extension service, university extensionan educational opportunity provided by colleges and universities to people who are not enrolled as regular students
(n)extension, filename extension, file name extensiona 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”
(n)reference, denotation, extensionthe most direct or specific meaning of a word or expression; the class of objects that an expression refers to“the extension of ‘satellite of Mars’ is the set containing only Demos and Phobos”
(n)extension, lengthiness, prolongationamount or degree or range to which something extends“the wire has an extension of 50 feet”
(n)extension, telephone extension, extension phonean additional telephone set that is connected to the same telephone line