God is a single being existing simultaneously as three distinct persons: The Father, The Son, and the Holy Spirit. The doctrine of the trinity is a reference to the "tri-unity" of God. There is only one God, but this God is a unity of one (Deuteronomy 6:4; Isaiah 44:6-8; 45:5-6; 1 Tim 2:5; 1 Cor 8:4). This God is also a complexity within unity (Gen. 2:24; 11:6; 1 Cor 3:6-8).
Scripture and tradition
The word "Trinity" comes from "Trinitas", a Latin abstract noun that most literally means "three-ness" (or "the property of occurring three at once"). Or, simply put, "three are one". The first recorded use of this Latin word was by Tertullian in about 200, to refer to Father, Son and Holy Spirit, or, in general, to any set of three things.