Hebrew for “accuser.” In Jewish tradition, you do not find the word by itself; rather it has the letter for the sound “ha” (meaning the article “the”) before it, clearly indicating that the name represents a position rather than a particular entity. “The Satan” could not do anything by itself, and is more like what would today be called a “devil’s advocate.”
In Christian theology, Satan became a specific entity, the representation of and cause of all that was bad, and the foe of God. Thus Christian theology became more dualistic in having a God of good (Jesus) and a God of evil (Satan) fighting over humans. This is closer to the concepts found in Gnosticism and Zoroastrianism.