Jesus said it was vision. What it was, was a preview of Christ's second coming. Before he took them up there He said,
28 Verily I say unto you, There be some standing here, which shall not taste of death, till they see the Son of man coming in his kingdom.
KJV Matthew 17:1 And after six days Jesus taketh Peter, James, and John his brother, and bringeth them up into an high mountain apart,
2 And was transfigured before them: and his face did shine as the sun, and his raiment was white as the light.
3 And, behold, there appeared unto them Moses and Elias talking with him.
4 Then answered Peter, and said unto Jesus, Lord, it is good for us to be here: if thou wilt, let us make here three tabernacles; one for thee, and one for Moses, and one for Elias.
5 While he yet spake, behold, a bright cloud overshadowed them: and behold a voice out of the cloud, which said, This is my beloved Son, in whom I am well pleased; hear ye him.
6 And when the disciples heard it, they fell on their face, and were sore afraid.
7 And Jesus came and touched them, and said, Arise, and be not afraid.
8 And when they had lifted up their eyes, they saw no man, save Jesus only.
9 And as they came down from the mountain, Jesus charged them, saying, Tell the vision to no man, until the Son of man be risen again from the dead. (Matt. 16:28-17:9 KJV)
Peter in his letter tells his readers that this was the power and coming of the Lord.
16 For we have not followed cunningly devised fables, when we made known unto you the power and coming of our Lord Jesus Christ, but were eyewitnesses of his majesty.
17 For he received from God the Father honour and glory, when there came such a voice to him from the excellent glory, This is my beloved Son, in whom I am well pleased.
18 And this voice which came from heaven we heard, when we were with him in the holy mount. (2 Pet. 1:16-18 KJV)
Peter said it was the power and coming of the Lord. Since this happened before Jesus went to the cross it surely wasn't His literal second coming. The second coming couldn't happen before Jesus had even left the first time. Also, note that they appeared din glory. This is a reference to the Resurrection. John told his readers that the Holy Spirit hadn't come yet because Jesus had not yet been glorified.
38 He that believeth on me, as the scripture hath said, out of his belly shall flow rivers of living water.
39 (But this spake he of the Spirit, which they that believe on him should receive: for the Holy Ghost was not yet given; because that Jesus was not yet glorified.) (Jn. 7:38-39 KJV)
Surely Jesus wasn't resurrected before He even went to the cross.