Ir.: "fairy sweetheart" or "fairy mistress"
A spirit--male or female--who is the lover of a mortal. Usually, the spirit is depicted as a woman. In Manx folklore, she (or he) is a vampiric spirit, sucking the lifeforce out of her (or his) lovers.
In Irish folklore, she is a muse of poetry, but a dangerous one--those who devote themselves to her live a short but glorious life. William Butler Yeats saw the leanan sidhe as the Dark Muse, an artistic succubus, giving creative gifts in exchange for the artist's life:
"Most of the Gaelic poets, down to quite recent times, have had a Leanhaun Shee, for she gives inspiration to her slaves and is indeed the Gaelic muse — this malignant fairy. Her lovers, the Gaelic poets, died young. She grew restless and carried them away to other worlds, for death does not destroy her power."
--W.B. Yeats, Fairy and Folk Tales of Ireland.
This, of course, is a well-known concept of the artist who dies young, often tragically: John Keats, Kurt Cobain; or who goes mad later in life (not unlike Yeats).
(Yeats' definition may have been just a bit influenced by his real-life muse, Maude Gonne.)
The idea of the dangerous fairy lover is also seen in later ballads such as "Tam Lin" or "Thomas the Rhymer." Particularly relevant is the story of Thomas the Rhymer, who reportedly gained his gift of prophecy from being the lover of the Queen of Elfland.