trinity (n.)

early 13c., "the Father, Son and Holy Spirit," constituting one God in prevailing Christian doctrine, from Old French trinite "Holy Trinity" (11c.), from Late Latin trinitatem (nominative trinitas) "Trinity, triad" (Tertullian), from Latin trinus "threefold, triple," from plural of trini "three at a time, threefold," related to tres (neuter tria) "three" (see three).

The Latin word was widely borrowed in European languages with the spread of Christianity (Irish trionnoid, Welsh trindod, German trinität). Old English used þrines as a loan-translation of Latin trinitas. Related: Trinitarian.