<filename extension> 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.
