The Collaborative International Dictionary of English v.0.48:

leprechaun \leprechaun\ n. (Irish folklore)
   A small mischevous elf or spirit in Irish folklore; it is
   often depicted in literature as a dwarfish bearded old man;
   -- legend tells that if a leprechaun is captured, he will
   reveal the location of his hidden pot of gold.
   [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

leprechaun
    n 1: a mischievous elf in Irish folklore

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "leprechaun":
   Ariel, Befind, Corrigan, Finnbeara, Mab, Oberon, Titania, banshee,
   brownie, cluricaune, dwarf, elf, fairy, fairy queen, fay, gnome,
   goblin, gremlin, hob, imp, kobold, ouphe, peri, pixie, pooka, puca,
   pwca, sprite, sylph, sylphid