berceuse

1. A lullaby.
2. A soothing musical composition, usually in 6/8 time.

[French, feminine of berceur, cradle rocker, from bercer, to rock, from Vulgar Latin *bertiāre.]

(Fr.)

‘Lullaby’: a quiet instrumental (especially piano) piece. Chopin's Berceuse in D♭ op.57 established compound metre, a ‘flat’ major key and simple, oscillating harmonies as its main features. See also Lullaby.

A lullaby.

The noun has one meaning:

Meaning #1: a quiet song intended to lull a child to sleep
  Synonyms: lullaby, cradlesong