Hadad (hā'dăd) [key], in the Bible. 1. Son of Ishmael. An alternate form is Hadar. 2. King of Edom. 3. Last king of Edom. Hadar is an alternate form. 4. Scion of the kings of Edom, who escaped Joab's massacre. He fled to Egypt and married the pharaoh's sister-in-law. Later he seems to have relieved Edom from Solomon's oppression.