“Substitutes, pl. of Badal. Certain persons by whom, it is said, God continues the world in existence. Their number is seventy, of whom forty reside in Syria, and thirty elsewhere. When one dies another takes his place, being so appointed by God. It is one of the signs of the last day that the Abdal will come from Syria. (Mishkat xxiii c. 3.) No one pretends to be able to identify these eminent persons in the world. God alone knows who they are and where they are.
Based on Hughes, Dictionary of Islam