Lewis & Short

dĕcennālis, e, adj. [decem-annus],

  1. I. of ten years, decennial (late Lat.; cf. decennis): bellum, i. e. Caesar’s in Gaul, Amm. 15, 12, 6; of the Trojan war, Hier. adv. Jovin. 1, 48.
  2. II. Subst.: DECENNALIA (ium, n.), a festival celebrated every ten years after the time of Augustus, Trebell. Gallien. 21, 5; Inscr. Grut. 116, 4; v. decennis fin.