Catechism of the Catholic Church

The Church uses (I) the term "substance" (rendered also at times by "essence" or "nature") to designate the divine being in its unity, (II) the term "person" or "hypostasis" to designate the Father, Son, and Holy Spirit in the real distinction among them, and (III) the term "relation" to designate the fact that their distinction lies in the relationship of each to the others.