Satan is a term, as well as the name of a figure appearing in the texts of the Abrahamic religions who brings evil and temptation, and is known as the deceiver that leads humanity astray. Some religious groups teach that he originated as an angel who fell out of favor with God, seducing humanity into the ways of sin, and who has power in the fallen world. In the Hebrew Bible and the New Testament, Satan is primarily an accuser and adversary, a decidedly malevolent entity (a.k.a. the devil) who possesses demonic qualities.
Source: Wikipedia, "Satan", available under the CC-BY-SA License.