There's a saying that English doesn't borrow from other languages. No, English bludgeons other languages over the head, drags them into dark alleys, then goes through their pockets for crumpled adjectives and loose grammar.
Most humor aside, English has so many parents that it would take an episode of Maury to sort out the father. With so many different languages thrown into a single melting pot it's no wonder that words which were once completely separate begin to bleed together until they're used interchangeably. This is what what happened to the words devil and demon. When you hear them you tend to think of the same images; creatures with red skin and horns, possibly bat wings, out to torment good people and to do harm. However, demons and devils are most definitely not the same thing.
Not by a long shot.
So What Is A Demon?
The word demon (spelled daemon in many variant translations) is a word that's actually older than the bibles it's typically used in. While it showed up in the year 1200 as part of Latin, it goes back even further to the Old Greek word daimon. How far back does it go? Well, at least as far back as Socrates.
The word daemon, at least in the time of the ancient Greeks, didn't have as many negative connotations as it does now. The word simply referred to a spirit, a divine power, or a lesser god. A genius loci (the guardian spirit of a physical place) might be considered a daemon, as would a protective ancestor spirit. Many men (including Socrates according to the Encyclopedia Britannica) had a personal daimon, which they often felt kept them safe and helped them achieve good things.
As with many terms used by pagan cultures after the spread of Christianity the idea of what a daemon or daimon was became cast in darkness. They became the unclean spirits of the world, and the word was often used to refer to beings who, while more powerful than men, were less powerful than god and his messengers.
Then What is a Devil?
The word devil comes from Old English, where the word was originally deofol, which meant an evil spirit or entity. This Old English word came from the Late Latin word diabolus, which is a decidedly different word from daemon. This is the first hint that demons and devils as we know them are not the same thing.
What does that all mean though?
Well, it means that while there are many demons, there is a devil. Singular. One.
Do You See The Difference?
Like most of the other linguistic mysteries concerning the bible the difference between devils and demons is nothing more than a simple translation error. While it might not be as cool as a conspiracy of renegade monks, or a deliberate cover up on the part of a heretical sect, the facts are still pretty interesting.
According to experts like Don Stewart the key is really in the Latin translations of the bible. While the Latin versions of the term devil and demon were both used quite liberally, they were never, every interchanged with one another. Diabolos was always used to refer to the devil, the Hebrew Satan, and never to any lesser spirits or servitors. So too when the word daemon was used it was never used to refer to Satan himself, but always to less beings and minions in a variety of different forms.
Some things just don't translate though. The shades of gray between these two words when the bible was translated into English got melted and merged, because in English an evil spirit is an evil spirit. English, as linguistic scholars and stand up comedians alike well know, just isn't good for being specific with just one word.