The King James Version uses devil to translate the Greek word (pronounced) dee-ab-ol-os (from which originated the English word diabolic) which literally means someone who attacks by means of slander or libel (see also "Run 'Em Through The Muck Righteousness") for Satan (the name Satan originated from the Hebrew word pronounced saw-tan which means adversary or accuser) and devils to translate the Greek word (pronounced) dah-hee-mon (from which originated the English word demon) for an evil spirit-being in general.
Examples of "devil" translating dee-ab-ol-os (i.e. diabolic) in referring to Satan:
"And the great dragon [see also Dragons] was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world [see also A Brainwashed World]: he was cast out into the earth, and his angels were cast out with him." (Revelation 12:9 KJV)
"Again, the devil taketh Him up into an exceeding high mountain, and showeth Him all the kingdoms of the world, and the glory of them; And saith unto Him, All these things will I give Thee, if Thou wilt fall down and worship me [see Could Jesus Have Sinned?]. Then saith Jesus unto him, Get thee hence, Satan: for it is written, Thou shalt worship the Lord thy God, and Him only shalt thou serve." (Matthew 4:8-10 KJV)
"Put on the whole armour of God, that ye may be able to stand against the wiles of the devil. For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world [see also Who Still Rules The World Today?, also Where Satan Dwells], against spiritual wickedness in high places." (Ephesians 6:11-12 KJV)
"He that committeth sin is of the devil; for the devil sinneth from the beginning. For this purpose the Son of God was manifested, that He might destroy the works of the devil [see also Why Is Christ Returning?]." (1 John 3:8 KJV)
Examples of "devils" translating dah-hee-mon (i.e. demons) in referring to evil angels:
"And Jesus went about all Galilee, teaching in their synagogues, and preaching The Gospel of The Kingdom, and healing all manner of sickness and all manner of disease among the people. And His fame went throughout all Syria: and they brought unto Him all sick people that were taken with divers diseases and torments, and those which were possessed with devils, and those which were lunatic, and those that had the palsy; and He healed them." (Matthew 4:23-24 KJV)
"And He was casting out a devil, and it was dumb. And it came to pass, when the devil was gone out, the dumb spake; and the people wondered. But some of them said, He casteth out devils through Beelzebub [see Baal-zebub and Beelzebub] the chief of the devils. And others, tempting Him, sought of Him a sign from heaven. But He, knowing their thoughts, said unto them, Every kingdom divided against itself is brought to desolation; and a house divided against a house falleth. If Satan also be divided against himself, how shall his kingdom stand? Because ye say that I cast out devils through Beelzebub. And if I by Beelzebub cast out devils, by whom do your sons cast them out? Therefore shall they be your judges. But if I with the finger of God cast out devils, no doubt the kingdom of God is come upon you." (Luke 11:14-20 KJV)
"And the seventy returned again with joy, saying, Lord, even the devils are subject unto us through thy name. And he said unto them, I beheld Satan as lightning fall from heaven. Behold, I give unto you power to tread on serpents and scorpions, and over all the power of the enemy: and nothing shall by any means hurt you." (Luke 10:17-19 KJV)
Fact Finder: What must be missing for someone to be possessed by a demon? How are demons cast out?
See Casting Out Demons