Meaning

noun the cardinal number that is the sum of one and one and one

noun the union of the Father and Son and Holy Ghost in one Godhead

noun three people considered as a unit

originFrom Middle English trinite, from Anglo-Norman trinite and Old French ternite (modern French trinité), from Latin trīnitās, from trīnī (“three each”), from trēs (“three”). By surface analysis, trine + -ity. Displaced native Old English þrines (literally “threeness”).

Etymology from Wiktionary, CC BY-SA 3.0