Noah Webster's Dictionary (a.) Not mutable; unchangeable; unalterable. Int. Standard Bible Encyclopedia IMMUTABILITY; IMMUTABLE i-mu-ta-bil'-i-ti, i-mu'-ta-b'-l (ametathetos): Occurs in Hebrews 6:17, 18 of the unchangeableness of the Divine counsel. It is the perfection of Yahweh that He changes not in character, will, purpose, aim (Malachi 3:6; so of Christ, Hebrews 13:8). See FAITHFULNESS; UNCHANGEABLE.

Multi-Version Concordance Immutable (1 Occurrence) Hebrews 6:18 that by two immutable things, in which it is impossible for God to lie, we may have a strong encouragement, who have fled for refuge to take hold of the hope set before us. (WEB KJV ASV WBS YLT)