In Norse mythology, a troll is a generally negative synonym for
jötunn. Later, in Scandinavian folklore, trolls became beings in
their own right. In modern times trolls are depicted in a variety
of media. In Norse mythology, troll, like thurs is a term applied
to jötnar. 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. There is much confusion and
overlap in the use of Old Norse terms jötunn, troll, þurs and risi.
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" and argues that since the sources disagree, it may be
most fruitful to investigate each text separately. Later, in
Scandinavian folklore, trolls become defined as a particular type
of being, generally held to be larger than humans and notably ugly.
Numerous tales about trolls are recorded, in which they are
frequently described as being extremely old, very strong, but slow
and dim-witted. They are sometimes described as man-eaters and as
turning to stone upon contact with sunlight. Into the 20th century,
the origins of particular Scandinavian landmarks, such as
particular stones, are ascribed to trolls. 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 explained as a
result of the "accuracy and efficiency of the lightning
strokes".