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