Ordovician

This term is a proper adjective and noun derived from the name of a specific geographic location in Wales where the strata were first studied. It is used exclusively within the fields of geology, paleontology, and stratigraphy to denote a precise window of Earth's history.

As a proper noun referring to a specific geologic period, it is treated as a singular entity. It does not have a plural form because there is only one Ordovician Period in the geologic time scale.