What is "filename extension"

n. (context computing English) A file extension.

n. a string of characters beginning with a period and followed by one to three 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, file name extension]

A filename extension is an identifier specified as a suffix to the name of a computer file. The extension indicates a characteristic of the file contents or its intended use. A file extension is typically delimited from the filename with a full stop (period).

Some file systems implement filename extensions as a feature of the file system itself, and may limit the length and format of the extension, while others treat filename extensions as part of the filename without special distinction.