Trinity: "The Christian understanding of God as one in essence though consisting of three distinct persons: Father, Son and Holy Spirit. The seeds of the doctrine lie in the New Testament witness that God reveals himself in three forms: as the Father who is the source of all things, as the divine Word who came in flesh to reveal the Father and redeem the fallen race, and as the Spirit who gives life and unity to the church and witnesses to the Father and divine Word. The theological terminology used by the church to express the doctrine has shifted meaning over time. The Greek fathers spoke of three hypostases (entities) in one ousia (being or substance), which became in Latin three personae in one substantia. The term for a person, however, in both Greek and Latin, did not carry the strong sense of individual self-consciousness found in the contemporary term."
Evans, C. (2002) Pocket Dictionary of Apologetics & Philosophy of Religion. Downers Grove, IL: InterVarsity Press.