The Collaborative International Dictionary of English v.0.48:

Peekaboo \Peek"a*boo\, n.
   A child's game; bopeep.
   [1913 Webster]

WordNet (r) 3.0 (2006):

peekaboo
    n 1: a game played with young children; you hide your face and
         suddenly reveal it as you say boo! [syn: peekaboo,
         bopeep]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "peekaboo":
   clear, clear as crystal, crystal, crystal-clear, crystalline,
   diaphane, diaphanous, filmy, gauzy, gossamer, gossamery,
   light-pervious, limpid, lucid, nonopaque, pellucid, revealing,
   see-through, sheer, thin, transparent, transpicuous, unclouded