a'-gag ('aghagh, or 'aghagh, meaning unknown, possibly "violent," BDB): A name, or title, applied to the king of the Amalekites, like Abimelech in Philistia and Pharaoh in Egypt. It is used of two of these kings: (1) A king of Amalek, mentioned by Balaam (Nu 24:7) in his blessing of Israel; (2) A later king, in the days of King Saul (1Sa 15:1-35). Saul was sent with his army to destroy the Amalekites, who had so violently opposed Israel in the Wilderness. He disregarded the Divine command, sparing the best of the spoil, and saving Agag the king alive (1Sa 15:8-9). After rebuking Saul, Samuel had Agag put to death for all the atrocities committed by himself and his nation (1Sa 15:32-33).
⇒See a list of verses on AGAG in the Bible.