Unitarianism (from Latin unitas "unity, oneness", from unus "one") is historically a Christian theological movement named for its belief that the God in Christianity is one entity, as opposed to the Trinity (tri- from Latin tres "three") which defines God as three persons in one being; the Father, Son, and Holy Spirit. Unitarian Christians, therefore, believe that Jesus was inspired by God in his moral teachings, and he is a savior, but he was not a deity or God incarnate. Unitarianism does not constitute one single Christian denomination, but rather refers to a collection of both extant and extinct Christian groups, whether historically related to each other or not, which share a common theological concept of the oneness nature of God.
While the uncompromising theological monotheism at the heart of Christian Unitarianism distinguishes it from the major Christian denominations which subscribe to Trinitarian theology, Christian Unitarianism is analogous to the more austere monotheistic understandings of God in Judaism, and to the concept of Tawhid (oneness of God) in Islam.
Unitarianism is also known for the rejection of several other Western Christian doctrines, including the doctrines of original sin, predestination, and the infallibility of the Bible. Unitarians in previous centuries accepted the doctrine of punishment in an eternal hell, but few do today.
Unitarianism might be considered a part of Protestantism, depending on one's stance or viewpoint, though usually it is excluded from that term due to its Nontrinitarian nature. Despite common origins during the Protestant Reformation, it is perhaps more accurate to call it a part of Nontrinitarianism. Some consider it both Protestant and Nontrinitarian, seeing no contradiction between those two terms. Nonetheless, this is not universally accepted.
Initially, the Unitarian movement was not called "Unitarian". It began almost simultaneously in the Polish–Lithuanian Commonwealth and in Transylvania in the mid-16th century. Among the adherents were a significant number of Italians who took refuge in Poland. In England, the first Unitarian Church was established in 1774 on Essex Street, London, where today's British Unitarian headquarters are still located. Since the theology was also perceived as deist, it began to attract many people from wealthy and educated backgrounds, although it was only at the late second half of the 18th century that it started to gain some wider traction within Christendom.
In the United States, Unitarianist theology spread first in New England, and the first official acceptance of the Unitarian faith on the part of a congregation in America was by King's Chapel in Boston, from where James Freeman began teaching Unitarian doctrine in 1784, and was appointed rector and revised the prayer book according to Unitarian doctrines in 1786. In J. Gordon Melton's Encyclopedia of American Religions, it is classified among "the 'liberal' family of churches".