Tschaneck (2030 m) – The last tree Traditional Cache

A cairn is a human-made pile (or stack) of stones. The word cairn comes from the Scottish Gaelic: càrn.

Cairns have been and are used for a broad variety of purposes, from prehistoric times to the present – all over the world.

In modern times, cairns are often erected as landmarks, a use they have had since ancient times. However, since prehistory, they have also been built and used as burial monuments; for defense and hunting; for ceremonial purposes, sometimes relating to astronomy; to locate buried items, such as caches of food or objects; and to mark trails, among other purposes.

They vary in size from small stone markers to entire artificial hills, and in complexity from loose conical rock piles to delicately balanced sculptures and elaborate feats of megalithic engineering. Cairns may be painted or otherwise decorated, whether for increased visibility or for religious reasons. An ancient example is the inuksuk (plural inuksuit), used by the Inuit, Inupiat, Kalaallit, Yupik, and other peoples of the Arctic region of North America. Inuksuit are found from Alaska to Greenland. This region, above the Arctic Circle, is dominated by the tundra biome and has areas with few natural landmarks.

The building of cairns for various purposes goes back into prehistory in Eurasia, ranging in size from small rock sculptures to substantial man-made hills of stone (some built on top of larger, natural hills). The latter are often relatively massive Bronze Age or earlier structures which, like kistvaens and dolmens, frequently contain burials; they are comparable to tumuli (kurgans), but of stone construction instead of earthworks. Cairn originally could more broadly refer to various types of hills and natural stone piles, but today is used exclusively of artificial ones.

The word cairn derives from Scots cairn (with the same meaning), in turn from Scottish Gaelic càrn, which is essentially the same as the corresponding words in other native Celtic languages of Britain, Ireland and Brittany, including Welsh carn (and carnedd), Breton karn, Irish carn, and Cornish karn or carn. Cornwall (Kernow) itself may actually be named after the cairns that dot its landscape, such as Cornwall's highest point, Brown Willy Summit Cairn, a 5 m (16 ft) high and 24 m (79 ft) diameter mound atop Brown Willy hill in Bodmin Moor, an area with many ancient cairns. Burial cairns and other megaliths are the subject of a variety of legends and folklore throughout Britain and Ireland. In Scotland, it is traditional to carry a stone up from the bottom of a hill to place on a cairn at its top. In such a fashion, cairns would grow ever larger. An old Scottish Gaelic blessing is Cuiridh mi clach air do chàrn, "I'll put a stone on your stone". In Highland folklore it is recounted that before Highland clans fought in a battle, each man would place a stone in a pile. Those who survived the battle returned and removed a stone from the pile. The stones that remained were built into a cairn to honour the dead. Cairns in the region were also put to vital practical use. For example, Dún Aonghasa, an all-stone Iron Age Irish hill fort on Inishmore in the Aran Islands, is still surrounded by small cairns and strategically placed jutting rocks, used collectively as an alternative to defensive earthworks because of the karst landscape's lack of soil.

In Scandinavia, cairns have been used for centuries as trail and sea marks, among other purposes. In Iceland, cairns were often used as markers along the numerous single-file roads or paths that crisscrossed the island; many of these ancient cairns are still standing, although the paths have disappeared. In Norse Greenland, cairns were used as a hunting implement, a game-driving "lane", used to direct reindeer towards a game jump.

In the mythology of ancient Greece, cairns were associated with Hermes, the god of overland travel. According to one legend, Hermes was put on trial by Hera for slaying her favorite servant, the monster Argus. All of the other gods acted as a jury, and as a way of declaring their verdict they were given pebbles, and told to throw them at whichever person they deemed to be in the right, Hermes or Hera. Hermes argued so skillfully that he ended up buried under a heap of pebbles, and this was the first cairn. In Croatia, in areas of ancient Dalmatia, such as Herzegovina and the Krajina, they are known as gromila.

In Portugal a cairn is called a moledro. In a legend the moledros are enchanted soldiers, and if one stone is taken from the pile and put under a pillow, in the morning a soldier will appear for a brief moment, then will change back to a stone and magically return to the pile. The cairns that mark the place where someone died or cover the graves alongside the roads where in the past people were buried are called Fiéis de Deus. The same name given to the stones was given to the dead whose identity was unknown. The Fieis de Deus or Fes de Deus are, in the Galician legends, spirits of the night. The word "Fes" or "Fieis" is thought to mean fairy, the same root as "fate" (fado), that can take the same meaning as the proto-Celtic *bato-, meaning "death".