Lewis & Short

Koppa (Coppa), n. indecl., = κόππα, a letter of the original Greek alphabet, in which it stood between π and ρ, and corresponded to the Hebr. koph and the Lat. q. It was rejected in the literary language, κ taking its place, but was retained as a numeral, = 90, Quint. 1, 4, 9; 1, 7, 10.