The Collaborative International Dictionary of English v.0.48:

babysitting \babysitting\ n.
   the work of a baby sitter; caring for children when their
   parents are not home.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

babysitting
    n 1: the work of a baby sitter; caring for children when their
         parents are not home [syn: babysitting, baby sitting]