Nouns

(n)extensiona mutually agreed delay in the date set for the completion of a job or payment of a debt“they applied for an extension of the loan”
(n)extensionact of expanding in scope; making more widely available“extension of the program to all in need”
(n)propagation, extensionthe spreading of something (a belief or practice) into new regions
(n)extension, extension service, university extensionan educational opportunity provided by colleges and universities to people who are not enrolled as regular students
(n)extensionact of stretching or straightening out a flexed limb
(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)extensionthe ability to raise the working leg high in the air“the dancer was praised for her uncanny extension”, “good extension comes from a combination of training and native ability”
(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
(n)elongation, extensionan addition to the length of something
(n)annex, annexe, extension, wingan addition that extends a main building