The Collaborative International Dictionary of English v.0.48:

babysitter \babysitter\, baby-sitter \baby-sitter\n.
   A person engaged to care for children when the parents are
   not home.

   Syn: sitter
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

babysitter
    n 1: a person engaged to care for children when the parents are
         not home [syn: babysitter, baby-sitter, sitter]