The term Trinity describes the central doctrine of Christianity - in God there are three distinct persons: Father, Son and Holy Spirit. The Athanasian creed (in use since the sixth century) describes it in this way:
"the Father is God, the Son is God, and the Holy Spirit is God, and yet there are not three Gods but one God."
Happy Second Sunday of Advent
"the Father is God, the Son is God, and the Holy Spirit is God, and yet there are not three Gods but one God."
Happy Second Sunday of Advent