The Collaborative International Dictionary of English v.0.48:

Sitter \Sit"ter\, n.
   1. One who sits; esp., one who sits for a portrait or a bust.
      [1913 Webster]

   2. A bird that sits or incubates.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Sitter
    n 1: Dutch astronomer who calculated the size of the universe
         and suggested that it is expanding (1872-1934) [syn:
         Sitter, Willem de Sitter]
    2: an organism (person or animal) that sits [ant: stander]
    3: a person engaged to care for children when the parents are
       not home [syn: babysitter, baby-sitter, sitter]
    4: a person who poses for a painter or sculptor [syn: artist's
       model, sitter]
    5: a domestic hen ready to brood [syn: brood hen, broody,
       broody hen, setting hen, sitter]