n. (context computing English) A file 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.