Sitter

from 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}]

from 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]