Fossil. Definition. noun, plural: fossils. (1) Any preserved evidence of life from a past geological age, such as the impressions and remains of organisms embedded in stratified rocks. (2) The mineralized remains of an animal or plant.adjective. Having the characteristic, or pertaining to the nature, of a fossil.1) Any preserved evidence of life from a past geological age, such as the impressions and remains of organisms embedded in stratified rocks. (2) The mineralized remains of an animal or plant. adjective. Having the characteristic, or pertaining to the nature, of a fossil.