Concise Bible Dictionary:
Three things are said to be unchangeable: God’s counsel, His word, and His oath (Heb. 6:17-18). To these certainty may be attached. In Malachi 3:6 we read, “I am Jehovah, I change not,” and the Lord is “the same yesterday, and to-day, and forever” (Heb. 13:8).
From Anstey’s Doctrinal Definitions:
This refers to God’s unchanging character—His unchangeableness (Mal. 3:6; Heb. 1:12; 13:8; James 1:17). It is an attribute essential to deity. In connection with the Lord’s dealings with Abraham, there are two things that are said to be unchangeable: the Word of God’s promise, and the oath that He made (Heb. 6:17-18).