The Collaborative International Dictionary of English v.0.48:

Silurian \Si*lu"ri*an\, a. [From L. Silures, a people who
   anciently inhabited a part of England and Wales.] (Geol.)
   Of or pertaining to the country of the ancient Silures; -- a
   term applied to the earliest of the Paleozoic eras, and also
   to the strata of the era, because most plainly developed in
   that country.
   [1913 Webster]

   Note: The Silurian formation, so named by Murchison, is
         divided into the Upper Silurian and Lower Silurian. The
         lower part of the Lower Silurian, with some underlying
         beds, is now separated under the name Cambrian, first
         given by Sedwick. Recently the term Ordovician has been
         proposed for the Lower Silurian, leawing the original
         word to apply only to the Upper Silurian.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Silurian \Si*lu"ri*an\, n.
   The Silurian age.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Silurian
    n 1: from 425 million to 405 million years ago; first air-
         breathing animals [syn: Silurian, Silurian period]