Definitions are a tricky thing. These discussions assume a foundational understanding of the Trinity. Here are a few basic definitions worth memorizing:
“Within the one Being that is God, there exists eternally three coequal and coeternal persons, namely, the Father, the Son, and the Holy Spirit”
The Westminster Shorter Catechism also has a helpful definition:
Q. 6. How many persons are there in the Godhead?
A. There are three persons in the Godhead: the Father, the Son, and the Holy Ghost;and these three are one God, the same in substance, equal in power and glory.
Another way of phrasing it is this:
When we are speaking of the Trinity we are speaking of one what and three who’s. The what is the Being or essence of God and the who’s are the Father, Son, and Spirit.
{James White’s book The Forgotten Trinity is an outstanding introduction}
