A lullaby is a song or folk poem meant to help a child fall asleep.

From A Poet’s Glossary

The following definition of the term lullaby is reprinted from A Poet's Glossary by Edward Hirsch.

A bedtime song or chant to put a child to sleep. Lullabies typically begin “Hush-a-bye baby, on the tree top,” or “Rock-a-bye, baby,” or “Sleep, my child,” or “Hush, little baby, don’t say a word.” The English term lullaby may derive from the sounds lu lu or la la, a sound that mothers and nurses make to calm babies, and by byor , another lulling sound or else a good-night term. The oldest lullaby to survive may be the lullaby of Roman nurses recorded in a scholium on Persius: “Lalla, Lalla, Lalla, / aut dormi, aut lacte” (Lullaby, Lullaby, Lullaby, / either go to sleep or suckle). As ancient folk poems, lullabies range from meaningless jingles to semi-ballads. They are closely related to nursery rhymes. Rodrigo Caro called these soothing melodies, which are found all over the world, the “reverend mothers of all songs.” Federico García Lorca noted that “Spain uses its very saddest melodies and most melancholy texts to darken the first sleep of her children” and concluded: “The European cradle song tries only to put the child to sleep, not, as the Spanish one, to wound his sensibility at the same time” (“On Lullabies,” 1928). Lorca reminds us that cradle songs were invented by women desperate to put their children to sleep. The women soothe their children by expressing their own weariness. The poems thus have a double purpose. He found the most ardent lullaby sung in Béjar and said, “This one would ring like a gold coin if we dropped it on the rocky earth.” It begins:

     Sleep, little boy,
     sleep, for I am watching you.
     God, give you much luck
     in this lying world.

Joseph Brodsky’s poignant late poem to his infant daughter, “Lullaby” (“Birth I gave you in a desert”) echoes one of W. H. Auden’s most beautiful early lyrics, “Lullaby” (“Lay your sleeping head, my love”).  Reetika Vazirani (1962–2003) wrote a startling and inconsolable three-line poem called “Lullaby” (2002), which wounds:

     I would not sing you to sleep.
     I would press my lips to your ear
     and hope the terror in my heart stirs you.

Read more from this collection.