Hypostatic union

The doctrine of the union of divine and human natures in Jesus Christ, without confusion of their respective substances.

From Greek Hypostasis — meaning “substance; subsistence;” from hypo “under, beneath” + stasis “a standing, a position”, (etymonline).

The hypostatic union describes how God the Son, while being fully God, took on a human nature at the same time. Jesus Christ has always had been God (John 8:58, 10:30), but became a human being at the incarnation (John 1:14). Jesus Christ, one Person, fully God and fully man -that there is no mixture or dilution of either nature, and that He is one united Person, forever.