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