“Hypostatic union” sounds fancy in English, but it’s actually a simple term. Hypostatic means personal. The hypostatic union is the personal union of Jesus’s two natures. Jesus has two complete natures: one fully human and one fully divine. What the doctrine of the hypostatic union teaches is that these two natures are united in one person in the God-man. Jesus is not two persons. He is one person. The hypostatic union is the joining (mysterious though it be) of the divine and the human in the one person of Jesus.
