gam·ma / ˈgamə/ • n. the third letter of the Greek alphabet (Γ, γ), transliterated as ‘g.’ ∎ [as adj.] denoting the third in a series of items, categories, etc. ∎ [as adj.] relating to gamma rays: gamma detector. ∎ Physics (pl. same) a unit of magnetic field strength equal to 10−5 oersted.