The condition of persons who are temporarily changed in appearance and nature—that is, lifted to a higher spiritual level—so that they can endure the presence and glory of heavenly beings.
Peter, James, and John saw the Lord glorified and transfigured before them. The Savior had before promised that Peter would receive the keys of the kingdom of heaven (Matt. 16:13–19; 17:1–9; Mark 9:2–10; Luke 9:28–36; 2 Pet. 1:16–18). At this important event, the Savior, Moses, and Elias (Elijah) gave the promised keys of the priesthood to Peter, James, and John. With these priesthood keys, the Apostles had the power to continue the work of the kingdom after the Ascension of Jesus.
Joseph Smith taught that on the Mount of Transfiguration, Peter, James, and John were also transfigured. They saw a vision of the earth as it will appear in its future glorified condition (D&C 63:20–21). They saw Moses and Elijah, two translated beings, and heard the voice of the Father. The Father said, “This is my beloved Son, in whom I am well pleased; hear ye him” (Matt. 17:5).