An imp is a small mythological being similar to a fairy or demon, frequently described in european folklore and fantasy:

An 'imp' is an off-shoot or cutting. Thus an 'ymp tree' was a grafted tree, or one grown from a cutting, not from seed. 'Imp' properly means a small devil, an off-shoot of Satan, but the distinction between goblins or bogles and imps from hell is hard to make, and many in the Celtic countries as well as the English Puritans regarded all fairies as devils, tom tit tot is called an impet. The fairies of tradition often hover uneasily between the ghostly and the diabolic state (1290: p.232).

Fantasy

The fictional universe of J.K.Rowling's 'Harry Potter' series, the imp is a magical beast classified as XX (harmless, may be domesticated):

The imp is found only in Britain and Ireland. It is sometimes confused with the pixie. They are of similar height (between six and eight inches), though the imp cannot fly as the pixie can, nor is it as vividly coloured (the imp is usually dark brown to black). It does, however, have a similar slapstick sense of humour. Its preferred terrain is damp and marshy, and it is often found near river banks, where it will amuse itself by pushing and tripping the unwary. Imps eat small insects and have breeding habits much like the fairies, though imps do not spin cocoons; the young are hatched fully formed at around one inch in length.

Fantastic Beasts and Where to Find Them (281: p.39)

In the English localization of books about the Witcher, small domestic demons hohliks from Polish and Belarusian folk-beliefs are translated as imps.