designating or of the second geologic period of the Paleozoic Era, characterized by the development of abundant marine invertebrates and the first vertebrates
See Ordovician in American Heritage Dictionary 4
(ôrˌdə-vĭshˈən)
adjective
Of or belonging to the geologic time, system of rocks, or sedimentary deposits of the second period of the Paleozoic Era, characterized by the appearance of primitive fishes. See Table at geologic time.