This is about Fido/Opus/SEAdog Standard Interface Layer. For the filesystem, see Fossil.
FOSSIL (Fido/Opus/SEAdog Standard Interface Layer) is a set of protocols for acting as an intermediary between bulletin-board-system software and the underlying hardware and systems software, allowing the BBS programs (and related things like network mail and file sending programs) to be implemented in a system-independent manner. Versions of the FOSSIL driver can be made for various system architectures. FOSSIL-based software was in wide use on FidoNet in the late 1980s and through the 1990s.