You were transfigured upon the mountain O Christ our God, showing to Your disciples Your glory as much as they could bear. Do also in us, sinners though we be, shine Your everlasting Light. At the intercessions of the Theotokos, O Giver of Light, glory to You.
Apolytikion (Festal Hymn) of the
Transfiguration of our Lord Jesus Christ

What does "transfiguration" mean?
Christ was transfigured, not by the addition of something He was not, nor by a transformation into something He was not, but by the manifestation to His disciples of what He really was.
He opened their eyes so that instead of being blind they could see. While He Himself remained the same, they could now see Him as other than He had appeared to them formerly. For He is ‘the true light’ (Jn 1:9), the beauty of divine glory, and He shone forth like the sun—though this image is imperfect, since what is uncreated cannot be imaged in creation without some diminution.
St. Gregory Palamas,
Archbishop of Thessaloniki
On the Transfiguration
from a homily by St Gregory Palamas
Jesus took Peter, James, and John, and led them up a high mountain where they were alone. There before them he was transfigured; his face shone like the sun.
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