In the context of BBSing, FOSSIL (Fido Opus Seadog Serial Interface Layer) is a standards protocol for creating an 8086/8088-compatible serial driver interface. It was primarily used to replace poor COM port support in the PC BIOS but used a similar (INT 14h) interface.
For Synchronet, a FOSSIL driver is needed to support 16-bit external BBS programs (i.e., door games), both for Windows and Unix.