The description (schema or language are valid terms here) that defines how to read and write a certain file.

Some file formats are open (eg: Adobe's PDF) and are easy to write software for. Some have been reverse engineered well, and some have been reverse engineered badly (Microsoft Word's .DOC).

See the Programmer's File Format Colletion

lib/main.php:944: Notice: PageInfo: Cannot find action page

lib/main.php:839: Notice: PageInfo: Unknown action