A. NAME Note: The adjective amethetos is translated in Heb 6:18 as a name: “immutability.” See B, No. 1. B. Adjectives 1. ametathetos (ajmetavqeto”, 276), adjective meaning immutable (a, privative; metatithemi, to change). It is used in Heb 6:18, where the “two immutable things” are the promise and the oath. In the V. 17 this word is used neuter with the article, as a noun, thereby denoting “immutability,” with reference to God’s counsel. Examples from the papyri show that it was used as a technical term in connection with wills. “The connotation adds much to the sense of Heb 6:17 (ff.)” (Moulton and Milligan).¶ 2. aparabatos (ajparavbato”, 531) is used of Christ’s priesthood, in Heb 7:24 “immutable” , unalterable, inviolable; the more literal meaning given in RVR77 and Besson, VM, as well as in the LBA margin: “non-transferable”, “non-transferable”, must be kept together with the first. The term includes both senses.¶
Source: Vine New Testament Dictionary