The Collaborative International Dictionary of English v.0.48:

Trias \Tri"as\, n. [L., triad. See Triad.] (Geol.)
   The formation situated between the Permian and Lias, and so
   named by the Germans, because consisting of three series of
   strata, which are called in German the Bunter sandstein,
   Muschelkalk, and Keuper.
   [1913 Webster]