From Latin fossilis, dug up.
In its early use, fossil was used to designate any rock, mineral or mineral substance dug out of the earth, but nowadays, it is used only in its restricted sense: it is the remains or traces of a plant or animal of a former geological period found in the strata of the earth. Figuratively, it is also used to talk about something that has lost its vitality or capacity for growth or progress, and it can be contemptuously applied to a person of antiquated notions or habits.
Shown below is one of Lyme Regis’ fine examples, spotted this weekend: an impressive ammonite revealed by erosion in a characteristic cliff face.