A troll is a supernatural being in Norse mythology and Scandinavian folklore. In origin, one of the meanings of the term troll was a negative synonym for a jötunn (plural jötnar), a being in Norse mythology, although the word was also used about witches, berserkers and various other evil magical figures. 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. In the Prose Edda book Skáldskaparmál, a scenario describing an encounter between an unnamed troll woman and the 9th century skald Bragi Boddason is provided. According to the section, once, late in the evening, Bragi was driving through "a certain forest" when a troll woman aggressively asked him who he was, in the process describing herself:
Anthony Faulkes translation:
John Lindow translation:
There is much confusion and overlap in the use of Old Norse terms jötunn, troll, þurs and risi, which describe various beings. Lotte Motz theorized that these were originally four distinct classes of beings; lords of nature (jötunn), mythical magicians (troll), hostile monsters (þurs) and heroic and courtly beings (risi)—the last class being the youngest addition. Ármann Jakobsson calls this theory "unsupported by any convincing evidence" . He has gone on to study the Old Norse examples of the term troll and has concluded that in the Middle Ages, the term is used to denote various beings such as a giant or mountain-dweller, a witch, an abnormally strong or large or ugly person, an evil spirit, a ghost, a blámaðr, a magical boar, a heathen demi-god, a demon, a brunnmigi or a berserk. 
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, unlike the rå and näck—who are attested as "solitary beings", trolls generally have "some form of social organization". Where they differ, Lindow adds, is that they are not Christian, and those that encounter them do not know them. Therefore trolls were in the end dangerous, regardless of how well they may get along with Christian society, and trolls display a habit of bergtagning ('kidnapping'; literally "mountain-taking") and overrunning a farm or estate.
While noting that the etymology of the word "troll" remains uncertain, John Lindow defines trolls in in later Swedish folklore as "nature beings" and as "all-purpose otherworldly being[s], equivalent, for example, to fairies in Anglo-Celtic traditions" and that they "therefore appear in various migratory legends where collective nature-beings are called for". Lindow notes that trolls are sometimes are swapped out for cats and "little people" in the folklore record.
A Scandinavian folk belief that lightning frightens away trolls and jötnar appears in numerous Scandinavian folktales, and may be a late reflection of the god Thor's role in fighting such beings. In connection, the lack of trolls and jötnar in modern Scandinavia is sometimes explained as a result of the "accuracy and efficiency of the lightning strokes". Additionally, the absence of trolls in regions of Scandinavia are described in folklore as being a "consequence of the constant din of the church-bells". This ring caused the trolls to leave for other lands, although not without some resistance; numerous traditions relate how trolls destroyed a church under construction or lunged boulders at stones at completed churches. Large local stones are sometimes described as the product of a troll's toss. Additionally, into the 20th century, the origins of particular Scandinavian landmarks, such as particular stones, are ascribed to trolls who may, for example, have turned to stone upon exposure to sunlight.
Lindow compares the trolls of the Swedish folk tradition to supernatural mead hall invader Grendel in the Old English poem Beowulf, and notes that "just as the poem Beowulf emphasizes not the harrying of Grendel but the cleansing of the hall of Beowulf, so the modern tales stress the moment when the trolls are driven off."
Smaller trolls are attested as living in burial mounds and in mountains in Scandinavian folk tradition. In Denmark, these creatures are recorded as troldfolk ("troll-folk"), bjergtrolde ("mountain-trolls"), or bjergfolk ("mountain-folk") and in Norway also as troldfolk ("troll-folk") and tusser. Trolls may be described as small, human-like beings or as tall as men depending on the region of origin of the story. James MacCulloch theorizes a connection between the Old Norse vættir and trolls, theorizing that both concepts may either stem from (or ultimately derive from) spirits of the dead.
In Norwegian tradition, similar tales may be told about the larger trolls and the Huldrefolk ("hidden-folk") yet a distinction is made between the two. The use of the word trow in Orkney and Shetland, to mean beings which are very like the Huldrefolk in Norway strongly suggests a common origin for the terms. The word troll may have been used by pagan Norse settlers in Orkney and Shetland as a collective term for supernatural beings who should be respected and avoided rather than worshiped. Troll could later have become specialized as a description of the larger, more menacing Jötunn-kind whereas Huldrefolk may have developed as the general term applied to smaller trolls.