Unitarianism is a Christian theological movement that believes that the God in Christianity is one entity, as opposed to a Trinity (tri- from Latin tres "three"). Most other branches of Christianity define God as one being in three persons: 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 is a specific theological movement started in the 16th century named after its distinctive non-Trinitarian belief that Jesus was just an inspired human rather than a divine incarnation of God himself. The movement started in Polish-Lithuanian Commonwealth and Transylvania and now includes several major denominations in both England and the US.

Biblical Unitarianism encompasses the key doctrines of Nontrinitarian Christians who affirm the Bible as their sole authority, and from it base their beliefs that God the Father is a singular being, the only one God, and that Jesus Christ is God’s son, but may not be divine. The term "biblical Unitarianism" is connected first with Robert Spears and Samuel Sharpe of the Christian Life magazine in the 1880s. It is a neologism (or retronym) that gained increasing currency in nontrinitarian literature during the 20th century as the mainstream Unitarian churches moved away from belief in the Bible and, in the United States, towards merger with Universalism. It has been used since the late 19th century by conservative Christian Unitarians, and sometimes by historians, to refer to Scripture-fundamentalist Unitarians of the 16th–18th centuries. Its use is problematic in that Unitarians from the 17th to the 20th centuries all had attachment to the Bible, but in differing ways.