In Arabian folklore, what is a Djinn?

The Djinn or jinn, also called a genie, in Arabic mythology, is a spirit inhabiting the earth but unseen by humans, capable of assuming various forms and exercising extraordinary powers. They are often conceptualized in Islam as creatures parallel to human beings who are capable of choosing between good and evil and must thus face eventual salvation or damnation. They are beings of smokeless flame by nature, in the same manner in which humans are said to be made of earth, and they cannot be seen by human beings.

Source: Encyclopedia Britannica