Immutable, Immutability - Vine's Expository Dictionary of New Testament Words
Immutable, Immutability[ 1,,G276, ametathetos ]
an adjective signifying immutable" (a, negative, metatithemi, "to change"), Hebrews 6:18, where the "two immutable things" are the promise and the oath. In Hebrews 6:17 the word is used in the neuter with the article, as a noun, denoting "the immutability," with reference to God's counsel. Examples from the papyri show that the word was used as a technical term in connection with wills, "The connotation adds considerably to the force of Hebrews 6:17 (and foll.)" (Moulton and Milligan).