Lewis & Short

occīdĭo, ōnis, f. [1. occīdo], a massacre, utter destruction, extermination (rare before the Aug. period; syn.: clades, strages, caedes): orare ne in occidione victoriam poneret, Liv. 3, 28.
Esp. freq. in the phrase occidione occidere or caedere, to cut off completely, cut down with utter destruction: equitatus occidione occisus, Cic. Fam. 15, 4, 7; id. Phil. 14, 14, 36; Liv. 28, 43; 2, 51; 9, 38 et saep.: omnes occidione caesi, Just. 26, 2, 5; 28, 2, 1; so, occidione occumbere, to be wholly cut off, Tac. A. 12, 38: equi, viri, cuncta victa occidioni dantur, every thing captured was destroyed, id. ib. 13, 57: occidioni exempti, id. ib. 12, 56 fin.
Of animals and plants: nec ad occidionem gens interimenda est, Col. 9, 15, 3: occidionem gregis prohibens, id. 7, 5, 16: nec ad occidionem universum genus perduci patimur, id. 4, 17, 3.