WordNet sense 1 (a string of characters beginning with a period and followed by one to three letters; the optional second part of a PC computer filename):

file name extension, filename extension

WordNet sense 2 (the spreading of something (a belief or practice) into new regions):

propagation

WordNet sense 3 (the most direct or specific meaning of a word or expression; the class of objects that an expression refers to):

denotation, reference

WordNet sense 4 (amount or degree or range to which something extends):

lengthiness, prolongation

WordNet sense 5 (an additional telephone set that is connected to the same telephone line):

extension phone, telephone extension

WordNet sense 6 (an addition to the length of something):

elongation

WordNet sense 7 (an addition that extends a main building):

annex, wing, annexe, annex

WordNet sense 8 (an educational opportunity provided by colleges and universities to people who not enrolled as regular students):

extension service, university extension