In the NT there is no substantial difference. Sometimes “unclean spirit” and “demon” occur in the same account, or in parallel accounts between the synoptic Gospel writers:
Luke 8:29 For he had commanded the unclean spirit to come out of the man. (For many a time it had seized him. He was kept under guard and bound with chains and shackles, but he would break the bonds and be driven by the demon into the desert.)
Luke 9:42 While he was coming, the demon threw him to the ground and convulsed him. But Jesus rebuked the unclean spirit and healed the boy, and gave him back to his father.
But in the OT the two are distinguished. Or rather “evil spirit” and “demons” are distinguished. All mentions of “evil spirit” come from God, but “demons” (in KJV below “devils”) are idols — which have no breath or spirit in them according to Jeremiah and Habakkuk.
The term “unclean spirit” is not used in the OT. But the idea of uncleanness of idols is found:
Zech. 13:2 “And on that day, declares the Lord of hosts, I will cut off the names of the idols from the land, so that they shall be remembered no more. And also I will remove from the land the prophets and the spirit of uncleanness.
There are three mentions of “evil spirit”, one a disagreement between men in Judges, the other two a form of mental illness that twice came upon Saul:
Jdg 9:23 (KJV) Then God sent an evil spirit between Abimelech and the men of Shechem; and the men of Shechem dealt treacherously with Abimelech:
1Sa 16:14 (KJV) But the Spirit of the LORD departed from Saul, and an evil spirit from the LORD troubled him. … 16 Let our lord now command thy servants, [which are] before thee, to seek out a man, [who is] a cunning player on an harp: and it shall come to pass, when the evil spirit from God is upon thee, that he shall play with his hand, and thou shalt be well.
1Sa 19:9 (KJV) And the evil spirit from the LORD was upon Saul, as he sat in his house with his javelin in his hand: and David played with [his] hand.
There is a third reference to a “lying spirit”, but that is in an ironic parable by the prophet Micaiah and best considered separately.
The Old Testament does not really know the concept of “demons”. Yes there are four verses which have traditionally been translated “devils” (KJV) or “demons”, but this is simply a description of idols — “who are no gods”.
Lev 17:7 (KJV) And they shall no more offer their sacrifices unto devils, after whom they have gone a whoring. This shall be a statute for ever unto them throughout their generations.
Deut 32:17 (KJV) They sacrificed unto devils, not to God; to gods whom they knew not, to new [gods that] came newly up, whom your fathers feared not.
2Ch 11:15 (KJV) And he ordained him priests for the high places, and for the devils, and for the calves which he had made.
Psa 106:37 (KJV) Yea, they sacrificed their sons and their daughters unto devils.
The OT also confirms that idols have no “spirit” (Hebrew: ruakh) in them:
Jer 10:14 (KJV) Every man is brutish in [his] knowledge: every founder is confounded by the graven image: for his molten image [is] falsehood, and there is no breath (spirit, ruakh) in them.
Jer 51:17 (KJV) Every man is brutish by his knowledge; every founder is confounded by the graven image: for his molten image [is] falsehood, and [there is] no breath [spirit, ruakh] in them.
Hab 2:19 (KJV) Behold, it is overlaid with gold and silver, And there is no breath (spirit) at all inside it
So the answer to the question is twofold
- in the NT there is no difference, both appear to exist in the 3 synoptic records — although as these are journalistic accounts of events they only reflect what was done and said. The gospel of John and the rest of the NT do not treat demons in this way.
- in the OT there is a big difference — evil spirits exist, and come from God, demons are just false gods who do not exist.
This raises another question : Why does OT and NT teaching on demons appear to differ?