The portion of a filename, following the final point, which indicates the kind of data stored in the file.

Many operating systems use filename extensions, e.g. unix, vms, ms-dos, microsoft windows. They are usually from one to three letters (some sad old OSes support no more than three). Examples include "c" for c source code, "ps" for postscript, "txt" for arbitrary text.

nextstep and its descendants also use extensions on directories for a similar purpose.

Apart from informing the user what type of content the file holds, filename extensions are typically used to decide which program to launch when a file is "run", e.g. by double-clicking it in a gui file browser. They are also used by unix's make to determine how to build one kind of file from another.

Compare: mime type.

tony warr's comprehensive list.

faqs.org graphics formats.