Source : Webster's Revised Unabridged Dictionary (1913)

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

   2. A bird that sits or incubates.

Source : WordNet®

Sitter
     n 1: Dutch astronomer who calculated the size of the universe and
          suggested that it is expanding (1872-1934) [syn: {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}]
     4: a person who poses for a painter or sculptor [syn: {artist's
        model}]
     5: a domestic hen ready to brood [syn: {brood hen}, {broody}, {broody
        hen}, {setting hen}]