A troll is a supernatural being in Norse mythology and Scandinavian folklore. In origin, troll may have been a negative synonym for a jötunn, a being in Norse mythology. In Old Norse sources, beings described as trolls dwell in isolated rocks, mountains, or caves, live together in small family units, and are rarely helpful to human beings.

Later, in Scandinavian folklore, trolls became beings in their own right, where they live far from human habitation, are not Christianized, and are considered dangerous to human beings. Depending on the region from which accounts of trolls stem, their appearance varies greatly; trolls may be ugly and slow-witted or look and behave exactly like human beings, with no particularly grotesque characteristic about them.

Trolls are sometimes associated with particular landmarks, which at times may be explained as formed from a troll exposed to sunlight. One of the most famous elements of Scandinavian folklore, trolls are depicted in a variety of media in modern popular culture.

In Norse mythology, troll, like thurs, is a term applied to jötnar, and are mentioned throughout the Old Norse corpus. In Old Norse sources, trolls are said to dwell in isolated mountains, rocks, and caves, sometimes live together (usually as father-and-daughter or mother-and-son), and are rarely described as helpful or friendly.

Later, in Scandinavian folklore, trolls become defined as a particular type of being. Numerous tales about trolls are recorded, in which they are frequently described as being extremely old, very strong, but slow and dim-witted, and are at times described as man-eaters and as turning to stone upon contact with sunlight. However, trolls are also attested as looking much the same as human beings, without any particularly hideous appearance about them, but where they differ is in that they live far away from human habitation, and trolls generally have "some form of social organization."