Real Dictionary

fossil
countable noun: A fossil is the hard remains of a prehistoric animal or plant that are found inside a rock.

fossil in American English
any rock or mineral dug out of the earth
of, having the nature of, or forming a fossil or fossils
noun: any remains, impression, or trace of a living thing of a former geologic age, as a skeleton, footprint, etc

fossil in British English
noun: a relic, remnant, or representation of an organism that existed in a past geological age, or of the activity of such an organism, occurring in the form of mineralized bones, shells, etc, as casts, impressions, and moulds, and as frozen perfectly preserved