Satan (n.)
proper name of the supreme evil spirit in Christianity, Old English Satan, from Late Latin Satan (in Vulgate in Old Testament only), from Greek Satanas, from Hebrew satan "adversary, one who plots against another," from satan "to show enmity to, oppose, plot against," from root s-t-n "one who opposes, obstructs, or acts as an adversary."

In Septuagint (Greek) usually translated as diabolos "slanderer," literally "one who throws (something) across" the path of another (see devil (n.)), though epiboulos "plotter" is used once.
In biblical sources the Hebrew term the satan describes an adversarial role. It is not the name of a particular character. Although Hebrew storytellers as early as the sixth century B.C.E. occasionally introduced a supernatural character whom they called the satan, what they meant was any one of the angels sent by God for the specific purpose of blocking or obstructing human activity. [Elaine Pagels, "The Origin of Satan," 1995]