From an LDS perspective, what is an Unclean Spirit?

I was surprised to find that I couldn't find a reference to this in the Bible Dictionary or on LDS org. There is a wiki article that treats the subject: http://www.mormonwiki.com/Evil_Spirits In agreement with this article, I have always been taught that an unclean spirit is a dis-embodied spirit corresponding to one of those that chose to follow Satan in the War in Heaven.

I'm not sure if you found your answer, but here is some information I have found in my own research.

Doctrine and Covenants Student Manual Section 129 Keys for Determining If Administrations Are from God https://www.lds.org/manual/doctrine-and-covenants-student-manual/sections-122-131/section-129-keys-for-determining-if-administrations-are-from-god?lang=eng

As I understand it, devils are spirits that were cast out of heaven for rebellion and are here on earth to tempt and torment man. Unclean spirits are disembodied spirits that once had bodies but are now in the spirit world. They were wicked in the flesh and unclean and therefore continue to serve their master, Satan. They are more powerful than devils because they have had bodies and understand the weaknesses of the flesh.

Evil Spirits and Unclean Spirits are not synonymous. Evil Spirits encompasses all of Satan’s followers, those who did not keep their first estate and those who do not keep their second estate.

In 3 Nephi 7:19 we learn Nephi “did he cast out devils AND unclean spirits”. This would indicate devils and unclean spirits are two different types of Evil Spirits.

Devils and demons are those who did not keep their first estate and were cast out of Heaven with Lucifer.

Unclean spirits also known as familiar spirits are disembodied spirits, meaning they once had a body and are now separated from it, kept their first estate, but followed Lucifer in their mortal life, second estate.

A disembodied spirit can not be one who was cast out of heaven with Lucifer because they never received a body to be disembodied from.

Unclean spirits are those spirit children of God who kept not their first estate, which refers to the life before earth life. All men and women lived with God as his spirit children before coming to the earth as mortal beings. Those who kept their first estate were rewarded with a chance to live on Earth, receive a mortal body, be resurrected, and obtain salvation. Those who keep their second estate (on Earth) shall be exalted. Unclean Spirits, or Evil spirits, are those who did not keep their first estate, and will never receive a mortal body.

The Book of Abraham gives us the most detailed information about our life before the creation of the Earth:

22 Now the Lord had shown unto me, Abraham, the intelligences that were organized before the world was; and among all these there were many of the noble and great ones;

23 And God saw these souls that they were good, and he stood in the midst of them, and he said: These I will make my rulers; for he stood among those that were spirits, and he saw that they were good; and he said unto me: Abraham, thou art one of them; thou wast chosen before thou wast born.

24 And there stood one among them that was like unto God, and he said unto those who were with him: We will go down, for there is space there, and we will take of these materials, and we will make an earth whereon these may dwell;

25 And we will prove them herewith, to see if they will do all things whatsoever the Lord their God shall command them;

26 And they who keep their first estate shall be added upon; and they who keep not their first estate shall not have glory in the same kingdom with those who keep their first estate; and they who keep their second estate shall have glory added upon their heads for ever and ever.

27 And the Lord said: Whom shall I send? And one answered like unto the Son of Man: Here am I, send me. And another answered and said: Here am I, send me. And the Lord said: I will send the first.

28 And the second was angry, and kept not his first estate; and, at that day, many followed after him.

That second angel, who was angry, and kept not is first estate, was Lucifer, Satan, the Devil, and those who followed after him; that third part who of the host of heaven who will never have a body of their own, and shall be cast our into outer darkness.

3 And there appeared another wonder in heaven; and behold a great red dragon, having seven heads and ten horns, and seven crowns upon his heads.

4 And his tail drew the third part of the stars of heaven, and did cast them to the earth: (Revelations 12:3-4)

...for, behold, the devil [...] rebelled against me, saying, Give me thine honor, which is my power; and also a third part of the hosts of heaven turned he away from me because of their agency; And they were thrust down, and thus came the devil and his angels; (Doctrine and Covenants 29:36-37)

And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day. (Jude 1:6)