Essence–energies distinction

In Eastern Orthodox (Palamite) theology, there is a distinction between the essence (ousia) and the energies (energeia) of God. It was formulated by Gregory Palamas (1296–1359) as part of his defense of the Athonite monastic practice of Hesychasm against the charge of heresy brought by the humanist scholar and theologian Barlaam of Calabria.