The Collaborative International Dictionary of English v.0.48:

Berceuse \Ber`ceuse"\, n. [F.] (Mus.)
   A vocal or instrumental composition of a soft tranquil
   character, having a lulling effect; a cradle song.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

berceuse
    n 1: a quiet song intended to lull a child to sleep [syn:
         lullaby, cradlesong, berceuse]