This is an important term, that follows with some very important concepts; the term is Theophany, lets explore it a bit.
Basic Definition -
An Appearance of God that is perceptible to Human Sight.
From the Greek Theos meaning God, and phainesthai meaning to appear.
Biblical Examples -
Genesis 12:7, Genesis 17:1, Genesis 18:1, Genesis 32:30, Exodus 6:2-3, Exodus 24:9-11, Exodus 33:11, Exodus 34:5, Numbers 12:6-8, Isaiah 6:1, Luke 3:22, Acts 7:2, And of Course Revelation.