The Greek adjective ametathetos means "that which cannot be changed" or "immutable." It combines the negative prefix a- with metatithemi (to transfer or alter), expressing absolute fixedness.
The double use of ametathetos in Hebrews 6:17-18 is one of Scripture's most powerful declarations of divine reliability. God's purpose is unchangeable; His oath is unchangeable. These two unchangeable things provide strong encouragement to hold on to the hope set before us. The God who promised cannot lie.