Lewis & Short

penthēmĭmĕres or -is, is, f., = πενθημιμερής (sc. τομή; consisting of five halves, i. e. of 2 1/2 parts), a part of a verse, consisting of its first two feet and a half, esp. of a hexameter or iambic trimeter, a penthemimeris, Aus. Ep. 4, 85 and 89; Diom. 497, 8; Ter. Maur. p. 2440 P.