1 The part of a file name that indicates the type of file. For example, in the file ‘document.txt’, the ‘txt’ is the extension, indicating that the file is a text file. A large number of such standard extensions exist. A list of common examples is given in the Appendices.
2 An add-on program that provides additional features to an existing program (for example, a QuarkXpress extension for indexing).
3 Of a source in coding theory, the process of encoding several symbols at a time, or the results thereof. If the symbols of a q-ary information source are taken r at a time, and the words of length r are treated (e.g. encoded) as if they were themselves symbols of an alphabet of size qr, then this compound source is called the rth extension of the original source. See also source coding theorem.