Source : Webster's Revised Unabridged Dictionary (1913)

Ordovician \Or`do*vi"cian\, a. [From L. Ordovices, a Celtic
   people in Wales.] (Geol.)
   Of or pertaining to a division of the Silurian formation,
   corresponding in general to the Lower Silurian of most
   authors, exclusive of the Cambrian. -- n. The Ordovician
   formation.

Source : WordNet®

Ordovician
     n : from 500 million to 425 million years ago; conodonts and
         ostracods and algae and seaweeds [syn: {Ordovician period}]