Four Character Codes, abbreviated as FourCC are 4 byte (32 bit) character sequences used as format identifiers in Microsoft's AVI (a.k.a. RIFF) container format. But it is also employed for the Video identifiers of other formats (because they also use 4 bytes) like Apple's QuickTime MOV container format. For MultimediaWiki's list of known video FourCCs see here.
Note! The audio counterpart to FourCC is called TwoCC.