The Lay of Leithian is an unfinished poem written by J. R. R. Tolkien. It tells the Tale of Beren and Lúthien, the story of the love of the mortal Man Beren and the immortal Elf maiden Lúthien. The poem consists of over 4200 verses. It was published after Tolkien's death in The Lays of Beleriand. Its precedents are found in the Welsh tale of Culhwch and Olwen in the manuscripts, the Red Book of Hergest and the earlier White Book of Rhydderch.
Leithian is a word in one of Tolkien's invented languages which derives from leithia, meaning "release". The title bears a strong resemblance to the word Leithien, the name for England in earlier versions of Tolkien's legendarium.
The author translated the title as release from bondage, without making explicit who was released from what form of bondage. There are, accordingly, several possible interpretations.
One likely meaning of the title might be found at one of the key moments in the poem, the point at which one of the Silmarils, the magical gems of Fëanor, is cut from the crown of Morgoth by Beren:
Behold! the hope of Elvenland
the fire of Fëanor, Light of Morn
before the sun and moon were born,
thus out of bondage came at last,
from iron to mortal hand it passed.
This moment is also central to the overarching story-line of The Silmarillion, in which the gem is used to bring hope to the scattered peoples of Middle-earth and is ultimately set in the heavens by the mariner Eärendil as a sign of their coming salvation.