Lewis & Short

dĕcennis, e, adj. [decem-annus],

  1. I. of ten years (post-Aug.): bellum, of the Greeks before Troy, Quint. 8, 4, 22; also proelium, Petr. 89, 2, 8: obsidio (Veiorum), Flor. 1, 12, 8: equae, ten years old, Plin. 8, 44, 69; cf.: decenni major asinus, Pall. Mart. 14, 1.
  2. II. Subst.: decennia, ium, n., = decennalia, a festival under the emperors, held every ten years, Treb. Gall. 5 fin.; cf.: decennalis, no. II.