Noah Webster's Dictionary (n.) The state or quality of being immutable; immutableness. 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 Immutability (1 Occurrence) Hebrews 6:17 In this way God, being determined to show more abundantly to the heirs of the promise the immutability of his counsel, interposed with an oath; (WEB KJV ASV WBS YLT)