(Latin: con, with; substantia, substance)
A translation of the Greek, homousios, chosen at the Council of Nicæa (A.D. 325) as the only correct word to express the nature of the Son of God.
He is not inferior to the Father, nor posterior, nor merely like unto Him, but identical in substance and in essence with Him.
He is truly God, God of very God, consubstantial with the Father, as the Nicene Creed has it, having, or rather, being, the Godhead no less than the Father.
New Catholic Dictionary