Lewis & Short

1. sēmestris, e, adj. [sex-mensis].

  1. I. Of six months, half-yearly, semi-annual: dies, nox, lasting six months (far in the north), Varr. R. R. 1, 2, 5: regnum, Cic. Att. 10, 8, 7: imperium, Caes. B. C. 1, 9: dux, Liv. 21, 43, 15: consulatus, Suet. Ner. 14: tribunatus (militaris), Plin. Ep. 4, 4, 2; hence also, aurum, the ring of the tribunes (worn for six months): semestri vatum digitos circumligat auro, Juv. 7, 89; cf. also, consilia, Suet. Aug. 35: spatium, Plin. 18, 29, 69, § 280: filius, six months old, id. 11, 51, 112, § 270; so, leones, id. 8, 16, 17, § 45; cf. vita, id. 8, 39, 60, § 141: aves, hirundines, id. 10, 25, 36, § 73.
  2. II. Plur. subst.: sē-mestrĭa, ium, n., the semi-annual collection of imperial ordinances, Dig. 2, 14, 46; 18, 7, 10 fin.; 29, 2, 12.