The Collaborative International Dictionary of English v.0.48:

Jurassic \Ju*ras"sic\, a. (Geol.)
   Of the age of the middle Mesozoic, about 190 to 140 million
   years ago, including, as divided in England and Europe, the
   Lias, O["o]lite, and Wealden; -- named from certain rocks of
   the Jura mountains. It was noted for the predominance of
   dinosaurs on land, and the development of the first birds and
   mammals. -- n. The Jurassic period or formation; -- called
   also the Jura.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

Jurassic
    adj 1: of or relating to or denoting the second period of the
           Mesozoic era
    n 1: from 190 million to 135 million years ago; dinosaurs;
         conifers [syn: Jurassic, Jurassic period]