Satan is the chief of the rebel angels. The anthropomorphic personification of evil, the Devil. In the judeo-christian tradition, Satan, from the hebrew word for 'adversary' (connoting the idea of oppostion), is the personification of evil and all that is hostile to God and his will.
In the Septuagint (Greek translation of the Hebrew Bible) the translation of 'satan' is given as 'diabolos' meaning 'Devil'. Both terms are usually employed synonymously. In the Old Testament Satan is presented as a distinct personality of darkness and accusation, a type of heavenly prosecutor. He is in charge of testing humans' integrity, by any means possible, but God has the authority to set limits to his power to do evil.
In the New Testament, he is pictured as a dragon or a serpent, described as the one who has the power of death, ruling with lies and deception, accusing humankind before God, and opposing the purpose of God in the world. Still, while his power is acknowledged, he is under God's rule; conceived as only part of creation, he could not overwhelm the Lord.