1680s, "one who rejects the doctrine of the Trinity," from Modern Latin unitarius (1650s), from Latin unitas (see unity) + -ian. Applied to Muslims and other non-Christian monotheists, but especially (and with a capital -u-) of a Christian body originally founded upon the doctrine of unipersonality. The American Unitarian Association formed in 1825. As an adjective from 1680s.
-ism
word-forming element making nouns implying a practice, system, doctrine, etc., from French -isme or directly from Latin -isma, -ismus (source also of Italian, Spanish -ismo, Dutch, German -ismus), from Greek -ismos, noun ending signifying the practice or teaching of a thing, from the stem of verbs in -izein, a verb-forming element denoting the doing of the noun or adjective to which it is attached. For distinction of use, see -ity. The related Greek suffix -isma(t)- affects some forms.