I am reading St. Augustine On the Holy Trinity. Augustine does not define his terms. But I do like what he says here:
"The Son of God is both understood to be equal to the Father according to the form of God in which He is, and less than the Father according to the form of a servant which He took; in which form He was found to be not only less than the Father, but also less than the Holy Spirit; and not only so, but less even than Himself...." - St. Augustine, 2.1.2
