The best definition I believe for demons is they are disembodied evil spirits.
Sorry, I may have misunderstood you here but, what is a "disembodied evil spirit"?
My view on these entities, that are 100% real, is that there are different categories of entities in this universe.
Other than the Father, Son and Holy Spirit that make up the Godhead.. all thing and beings are created.
Of the beings that were created, there are animals and humans that live in our dimension, here on earth. Bugs, insects, fish.... we all are hatched or born.. live and die and our bodies decay.
Then there are heavenly beings.. We call them Angels but "angels" are just one "class".
There are Cherubim, Seraphim, Principalities, Authorities, and others.
Of these, which all have different ranks, duties and abilities.. there are those that sided with Satan and were thrown out of heave. Actually, this made up one third of the total number of these heavenly beings.
The other two thirds remained faithful and honoring to their creator.. God.
The entry of "demons" is, in my opinion and with good reason and support, is the result of the Genesis 6 events.
Somehow, and with great debate around the exact details, there were "nephilim" created by the mating of "the sons of God" and the "daughters of men".
These were the giants and "men of renown". These were, again IMO, not pure human. It is my view that it was the hybridization of angles and human women.
Fallen angels, under direction of Satan, mated with human women and produced angel/human hybrids.
Due to them, no matter who you believe to be the fathers.. were not able to receive salvation. This topic is a very deep and debated subject.
In the end... "demons" are the souls of these hybrids after their deaths. They only show up in the bible after the flood. They are condemned to roam the earth, and whatever dimension they abide in, without bodies. However, they still posses their appetites for sex, thirst, hunger and other desires that they would have had during their earthly existence.
Jude talks of the angles, that did this nasty deed, in his book as those that "left their first estate" and are chained in hell.
This is what, IMO, demons are and they are real and they desire to posses any body that they are able to by permission and actions of the person.
I believe 100% that they exist and posses people today.
Sometimes it may be very obvious. Other times it may be less so.
If you really want to delve into this subject and how it is affecting the churches and people of the world today... look into the "spirit of Jezebel"