The Collaborative International Dictionary of English v.0.48:

Saint Nicholas \Saint Nicholas\, St Nicholas \St. Nicholas\, n.
   A Dutch saint, who was reputed to bring gifts to children on
   Christmas even, giving rise to the modern legend of Santa
   Claus.
   [PJC]

   A Visit from St. Nicholas The original name for a poem by
      Clement Clarke Moore, popularly called titled The Night
      Before Christmans. It is a popular poem with the theme of
      St. Nicholas (Santa Claus) coming to bring gifts to
      children on Christmans eve. See Night Before Christmas
      in the vocabulary.
      [PJC]

WordNet (r) 3.0 (2006):

St. Nicholas
    n 1: a bishop in Asia Minor who is associated with Santa Claus
         (4th century) [syn: Nicholas, Saint Nicholas, St.
         Nicholas]