The format of a file,
the language in which a file
is written: how is the information stored in the file: encoding, grammar rules, semantics.

Examples are GIF, PNG, PostScript, HTML, Microsoft Word ".doc" format.

File formats can be kind of proprietary, in that vendors can
use them to trap people and organisations into vendor lock-in.
Having standards for file formats is
often a good remedy, especially if the standard is maintained