Satan is often used to refer to the "angel who fell out of favor with God." Satan was employed by God to tempt man. It wasn't until the New Testament that he is regarded as malevolent.
Satan is a character appearing in the texts of the Abrahamic religions, who personifies evil and temptation, and is known as the deceiver that leads humanity astray. The term is often applied to an angel who fell out of favor with God, seducing humanity into the ways of sin, and who now rules over the fallen world.