Lewis & Short

trĭmĕtrŏs or -trus, tra, trum (postclass. collat. form trĭmĕtrĭus, a, um, Aus. Ep. 16, 78; Sid. poët. Ep. 9, 15), adj., = τρίμετρος; in prosody, containing three metres or double-feet, trimeter: versus, Quint. 10, 1, 99.
As subst.: trĭmē̆tros (-trus), i, m., a trimeter, Quint. 9, 4, 90; 9, 4, 75; 9, 4, 108 al.; Hor. A. P. 252; Ter. Maur. p. 2432 P.; Diom. p. 506 ib.
Also called trĭmĕ-ter, Serv. Verg. E. 8, 78.