A theophany is a visible manifestation of God usually restricted to the Old Testament. God has appeared in dreams (Gen. 20:3-7, Gen. 28:12-17), visions (Gen. 15:1-21, Isaiah 6:1-13), as an angel (Gen. 16:7-13, 18:1-33), etc.
There is a manifestation known as the Angel of the Lord (Judges 6:20f.) and seems to have characteristics of God Himself (Gen. 16:7-9, 18:1-2, Exodus 3:2-6, Joshua 5:14, Judges 2:1-5, 6:11). Such characteristics as having the name of God, being worshipped, and recognized as God has led many scholars to conclude that the angel of the Lord is really Jesus manifested in the Old Testament. This does not mean that Jesus is an angel. The word "angel" means messenger. Other Scriptures that describe more vivid manifestations of God are Gen. 17:1, 18:1, Ex. 6:2-3, 24:9-11, 33:20, Num. 12:6-8, Acts 7:2. For further information on theophanies, see the Plurality Study.