filename extension

A filename extension is a suffix to the name of a computer file applied to indicate its type. It is commonly used to infer information about what sort of data might be stored in the file. The description above is meant to mostly explain the intent of filename extensions: a true definition, giving the criterion for deciding what part of the file name is its extension, belongs to the rules of the specific filesystem used; most times the extension is the substring which follows the last occurrence, if any, of the dot character (e.g. "txt" is the extension of the filename "readme.txt", "html" the extension of "mysite.index.html"). On filesystems on mainframe systems such as MVS, VMS, and PC systems such as CP/M and derivative systems such as Microsoft DOS, the extension is actually a separate namespace from the filename. This is different from Unix-like operating systems, where filesystems do not actually support the notion of an extension, where a suffix is not a separate namespace, and where even having a suffix is voluntary for executables, as permissions are used to decide whether a file is executable.
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<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, 

Noun
1. 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"
(synonym) extension, file name extension
(hypernym) string
(part-holonym) filename, file name, computer filename, computer file name