A filename or file name is a name used to uniquely identify a computer file in a file system. Different file systems impose different restrictions on filename lengths.

A filename may (depending on the file system) include:

• name – base name of the file
• extension – may indicate the format of the file (e.g. .txt for plain text, .pdf for Portable Document Format, .dat for unspecified binary data, etc.)

The components required to identify a file by utilities and applications varies across operating systems, as does the syntax and format for a valid filename.

Filenames may contain any arbitrary bytes the user chooses. This may include things like a revision or generation number of the file such as computer code, a numerical sequence number (widely used by digital cameras through the DCF standard), a date and time (widely used by smartphone camera software and for screenshots), and/or a comment such as the name of a subject or a location or any other text to facilitate the searching the files. In fact, even unprintable characters, including bell, 0x00, Return and LineFeed can be part of a filename, although most utilities do not handle them well.

Some people use of the term filename when referring to a complete specification of device, subdirectories and filename such as the Windows C:\Program Files\Microsoft Games\Chess\Chess.exe. The filename in this case is Chess.exe. Some utilities have settings to suppress the extension as with MS Windows Explorer.