Hesychast monks reported seeing a divine light in prayer. When their claims were attacked, Gregory Palamas argued that it was the same uncreated light that the apostles saw on Mount Tabor, an energy of God, not a created phenomenon, and that seeing it was a participation in God.
Uncreated light
The divine light seen by the hesychasts, identified with the light of Christ’s Transfiguration on Mount Tabor.
Also: Taboric light, Light of Tabor