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.
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