sitter

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One that sits, especially.
• noun A person who cares for young children when the parents are not home; a babysitter.
• noun A person who poses or models, as for a portrait.
• noun A brooding hen.

from The Century Dictionary.

• noun In cricket: An easy catch missed by a fielder.
• noun A fielder who misses such a catch.
• noun One who or that which sits
• noun A brooding or incubating bird.
• noun One who takes a certain posture, position, or course in order to a particular end; specifically, one who poses to an artist for a portrait, bust, or the like.
• noun One who watches during the night.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who sits; esp., one who sits for a portrait or a bust.
• noun A bird that sits or incubates.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Someone who sits e.g. for a portrait.
• noun One employed to watch or tend something; the general form of babysitter, housesitter, petsitter, etc.
• noun A broody hen.
• noun A really easy scoring chance.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a person engaged to care for children when the parents are not home
• noun a domestic hen ready to brood
• noun Dutch astronomer who calculated the size of the universe and suggested that it is expanding (1872-1934)
• noun an organism (person or animal) that sits
• noun a person who poses for a painter or sculptor