Wiktionary
n. (context archaic idiomatic English) A long-lasting agreement.
Wikipedia
Covenant of salt is a description of God's covenant used on three occasions in the Hebrew bible:
The commandments regarding grain offerings in the Book of Leviticus state "every offering of your grain offering you shall season with salt; you shall not allow the salt of the covenant of your God to be lacking from your grain offering. With all your offerings you shall offer salt."
In the Book of Numbers, God's covenant with the Aaronic priesthood is said to be a covenant of salt.
In the second book of Chronicles, God's covenant with the Davidic kings of Israel is also described as a covenant of salt.
Salt is also used as a symbol of covenant-making in the Arabic expression 'there is salt between us'
Usage examples of "covenant of salt".
Ought ye not to know that the LORD God of Israel gave the kingdom over Israel to David for ever, even to him and to his sons by a covenant of salt?
Ought you not to know that the LORD God of Israel gave the kingship over Israel for ever to David and his sons by a covenant of salt?