In Nordic folklore, including the Northern Isles, hidden-folk (Faroese and Icelandic: huldufólk; Norwegian: huldrefolk), mound-folk (Danish: højfolk, Swedish: högfolk), mountain-folk (Danish: bjærgfolk, Swedish: bergfolk, bergafolk; Finnish: vuoren väki), subterraneans (Norwegian: underjordiske, Swedish: underjordiska; Gutnish: di sma undar jordi, "the small underground"; Finnish: maahiset, roughly "grounders"), among other names (Swedish: oknytt, Insular Scots: trow, drow), are collective names for a loose race or conglomeration of elves, wights, brownies (Nordic: nisse), trolls, and thereof. They are supernatural beings that live in nature. They look and behave similarly to humans, but live in a parallel world. They can make themselves visible at will. Konrad von Maurer cites a 19th-century Icelandic source claiming that the only visible difference between normal people and outwardly human-appearing huldufólk is, the latter have a convex rather than concave philtrum (Icelandic: vuldulág) below their noses. An analog myth are the trolls being discerned by their large noses.

In Faroese folk tales, hidden people are said to be "large in build, their clothes are all grey, and their hair black (the same description is also given for draugrs in Faroese folklore). Their dwellings are in mounds, and they are also called Elves." Some Icelandic folk tales caution against throwing stones, as it may hit the hidden people.

The term huldufólk was taken as a synonym of álfar (elves) in 19th-century Icelandic folklore. Jón Árnason found that the terms are synonymous, except álfar is a pejorative term. Konrad von Maurer contends that huldufólk originates as a euphemism to avoid calling the álfar by their real name.

There is, however, some evidence that the two terms have come to be taken as referring to two distinct sets of supernatural beings in contemporary Iceland. Katrin Sontag found that some people do not differentiate elves from hidden people, while others do. A 2006 survey found that "54% of respondents did not distinguish between elves and hidden people, 20% did and 26% said they were not sure."

There are countless names for the phenomenon around the Nordics and beyond, most of which are purely descriptive at their core. A modern example can be found in the children's book character Plupp, by Swedish author Inga Borg, who according to the author is an "invisibling" (osynling, lit. 'unseenling').

Icelandic and Faroese Huldufólk, as well as Norwegian: huldrefolk, means "hidden-folk", compounding huldu/huldre- ("pertaining to secrecy"), and fólk ("people, folk"). The fore element stems from Old Norse: huldr, past participle of hylja ("to hide, cover"). Beings within huldufólk may be called "hulder" m., or "huldra" f., etc.

Related terms includes hulder, a related entity, and in Old Norse poetry, related forms can also be found in huldarhǫttr (Danish: hyllehat, Norwegian: hulderhatt) and huliðshjalmr, "hat-" and "helmet of invisibility" respectively. In Huldar saga, Huldr is the name of a witch.

In Finnish, hidden folk are called maahinen (plural: maahiset), compounding Finnish: maa ("ground, land"), with the suffix -hinen, which forms nouns or adjectives of belonging to or possessing a particular quality. If a maahinen is in a forest, it could be called metsähinen instead, and if in water, vetehinen. The Tornedalian form is maahiainen.