FOSSIL is a standard protocol for allowing serial communication for telecommunications programs under the DOS operating system. FOSSIL is an acronym for Fido Opus SEAdog Standard Interface Layer. Fido refers to FidoBBS, Opus refers to Opus-CBCS BBS, and SEAdog refers to a Fidonet compatible mailer. The standards document that defines the FOSSIL protocol is maintained by the Fidonet Technical Standards Committee.

Fossils (from Classical Latin fossilis; literally, "obtained by digging") are the preserved remains or traces of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous (fossil-containing) rock formations and sedimentary layers (strata) is known as the fossil record.