The Collaborative International Dictionary of English v.0.48:

Devonian \De*vo"ni*an\, a. (Geol.)
   Of or pertaining to Devon or Devonshire in England; as, the
   Devonian rocks, period, or system.
   [1913 Webster]

   Devonian age (Geol.), the age next older than the
      Carboniferous and later than the Silurian; -- called also
      the Age of fishes. The various strata of this age
      compose the Devonian formation or system, and include the
      old red sandstone of Great Britain. They contain, besides
      plants and numerous invertebrates, the bony portions of
      many large and remarkable fishes of extinct groups. See
      the Diagram under Geology.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Devonian \De*vo"ni*an\, n.
   The Devonian age or formation.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Devonian
    n 1: from 405 million to 345 million years ago; preponderance of
         fishes and appearance of amphibians and ammonites [syn:
         Devonian, Devonian period, Age of Fishes]