In Eastern Orthodox theology, the Tabor Light (also Light of Tabor, Tabor’s Light, Taboric Light; Greek: Φῶς του Θαβώρ, also as Ἄκτιστον Φῶς, Uncreated Light, Θεῖον Φῶς, Divine Light; Russian: Фаворский свет) is the light revealed on Mount Tabor at the Transfiguration of Jesus, identified with the light seen by Paul at his conversion.
As a theological doctrine, the uncreated nature of the Light of Tabor was formulated in the 14th century by Gregory Palamas, an Athonite monk, defending the mystical practices of Hesychasm against accusations of heresy by Barlaam of Calabria. When considered as a theological doctrine, this view is known as Palamism after Palamas.
The view was very controversial when it was first proposed, sparking the Hesychast controversy, and the Palamist faction prevailed only after the military victory of John VI Kantakouzenos in the Byzantine civil war of 1341–1347. Since 1347, it has been the official doctrine in Eastern Orthodoxy, while it remains without explicit affirmation or denial by the Roman Catholic Church. Roman Catholic theologians have rejected it in the past, but Andreas Andreopoulos has said that now “the Western world has started to rediscover what amounts to a lost tradition. Hesychasm, which was never anything close to a scholar’s pursuit, is now studied by Western theologians who are astounded by the profound thought and spirituality of late Byzantium.” Speaking of the hesychast controversy, Pope John Paul II said the term “hesychasm” refers to a practice of prayer marked by deep tranquillity of the spirit intent on contemplating God unceasingly by invoking the name of Jesus. While from a Catholic viewpoint there have been tensions concerning some developments of the practice, the Pope said, there is no denying the goodness of the intention that inspired its defence, which was to stress that man is offered the concrete possibility of uniting himself in his inner heart with God in that profound union of grace known as theosis, divinization.
Source: Wikipedia: Uncreated Light