Trinitarianism
From ReformedWiki.org
Trinitarianism is the doctrine that God exists as three co-equal, co-eternal persons—Father, Son, and Holy Spirit—within one divine essence. This belief emphasizes the unity and distinctiveness of each person of the Trinity, affirming that the Father is the source of all things, the Son is the incarnate Word who accomplished redemption, and the Holy Spirit is the giver of life who applies the benefits of Christ's work to the elect. The Trinity is foundational to understanding God's nature and His work in creation, redemption, and sanctification, with each person of the Godhead playing a distinct yet harmonious role in the salvation of believers.
Nontrinitarianism is the rejection of the trinity, and is a heresy.