Hebrew for "adversary". Judaism casts this fellow as a sort of heavenly D.A., charged with pointing out people's sins. (q.v. the Book of Job.) Hence, perhaps, the modern expression "devil's advocate".

In Christianity the idea of Satan became conflated with several other mythological figures:

As a result, we now have this image of Satan as a sunburnt fallen angel with goat feet, ruling over hell. It's a total pastiche, but then, Western culture is good at pastiche.

Some medieval mages and alchemists claimed to be able to summon Satan (amongst other angels and demons) safely, without harm to their souls, on account of their status as Christians. Later, this practice became considered demonolatry, or demon worship, and was condemned by the Inquisition among others. "Consorting with demons" (which meant fucking them as well as worshipping them) was a charge laid against supposed witches during the Burning Times.

Later, when magick underwent a bit of a revival in the Renaissance and after, it was believed that summoning Satan or demons was characteristic of the left-hand path (or black magick), and white magickians would have nothing to do with it.

Today, we have Satanists of several ilk: some are demonolaters, while others are simply black magicians using the image Satan as a ritual focus, and others are atheist anarchists who identify with the image of Satan as rebel.

(Satan Adama Tabat Amada Natas.)