Concise Bible Dictionary:
A title of God used by Daniel, alluding to His eternity. It cannot be separated from Christ; for in Dan. 7 the Lord is called both the Ancient of Days and the Son of Man, yet the Son of Man came to the Ancient of Days to receive dominion, glory, and a kingdom (Dan. 7:9,13,22). He is both God and man; compare Revelation 1 and 5.