A filename is 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:

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.

The characters allowed in filenames depend on the file system. The letters A–Z and digits 0–9 are allowed by most file systems; many file systems support additional characters, such as the letters a–z, special characters, and other printable characters such as accented letters, symbols in non-Roman alphabets, and symbols in non-alphabetic scripts. Some file systems allow even unprintable characters, including Bell, Null, Return and Linefeed, to be part of a filename, although most utilities do not handle them well.

Filenames may include things like a revision or generation number of the file, 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), or a comment such as the name of a subject or a location or any other text to help identify the file.

Some people use 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.^{[not verified in body]}

During the 1970s, some mainframe and minicomputers had operating systems where files on the system were identified by a user name, or account number.

For example, on the TOPS-10 and RSTS/E operating systems from Digital Equipment Corporation, files were identified by