The Collaborative International Dictionary of English v.0.48:

lullaby \lull"a*by\, n. [From Lull, v. t. ]
   1. A song to quiet babes or lull them to sleep; that which
      quiets. --Shak.
      [1913 Webster]

   2. Hence: Good night; good-by. [Obs.] --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

lullaby
    n 1: a quiet song intended to lull a child to sleep [syn:
         lullaby, cradlesong, berceuse]
    2: the act of singing a quiet song to lull a child to sleep
       [syn: lullaby, cradlesong]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "lullaby":
   Schlummerlied, Wiegenlied, anesthetic, berceuse, cradlesong,
   hypnotic, mandrake, morphia, morphine, nightcap, opiate, opium,
   poppy, sedative, sleep-bringer, sleep-inducer, sleep-producer,
   somnifacient, soporific