The Collaborative International Dictionary of English v.0.48:

Trimeter \Trim"e*ter\, a. [L. trimetrus, Gr. ?; ? (see Tri-) +
   ? measure. See Meter measure.] (Pros.)
   Consisting of three poetical measures. -- n. A poetical
   division of verse, consisting of three measures. --Lowth.
   [1913 Webster]