A word not found in Scripture, but used to express the doctrine of the unity of God as subsisting in three distinct Persons. This word is derived from the Gr. trias, first used by Theophilus (A.D. 168-183), or from the Lat. trinitas, first used by Tertullian (A.D. 220), to express this doctrine. The propositions involved in the doctrine are these: 1. That God is one, and that there is but one God (Deuteronomy 6:4; 1 Kings 8:60; Isaiah 44:6; Mark 12:29,32; John 10:30). 2. That the Father is a distinct divine Person (hypostasis, subsistentia, persona, suppositum intellectuale), distinct from the Son and the Holy Spirit. 3. That Jesus Christ was truly God, and yet was a Person distinct from the Father and the Holy Spirit. 4. That the Holy Spirit is also a distinct divine Person.
These dictionary topics are from M.G. Easton M.A., D.D., Illustrated Bible Dictionary, Third Edition, published by Thomas Nelson, 1897. Public Domain, copy freely.
Easton, Matthew George. "Entry for 'Trinity'". "Easton's Bible Dictionary".