Duck, any of various species of relatively small, short-necked, large-billed waterfowl. In true ducks—i.e., those classified in the subfamily Anatinae in the waterfowl family Anatidae—the legs are placed rearward, as in swans, rather than forward, as in geese. The result is a distinctive waddling gait. Most true ducks, including a few inaccurately called geese (e.g., sheldgeese) by reason of size and build, also differ from swans and true geese in the following characteristics: males (drakes) and females (hens or ducks) exhibit some degree of differentiation in plumage and in call, males molt twice annually, females lay large clutches of smooth-shelled rather than rough-shelled eggs, and both sexes have overlapping scales on the skin of the leg. The wild mallard (Anas platyrhynchos) is believed to be the ancestor of all domestic ducks, and it has undergone numerous crossbreedings and mutations since it was first domesticated in China between 2,000 and 3,000 years ago.

All true ducks, except those in the shelduck group (the tribe Tadornini) and sea ducks (the tribes Mergini and Somateriini), mature in the first year and pair only for the season—unlike the late-maturing, life-mating true geese and swans. Ducks are generally divided into three major groups, dabbling (shallow-water), diving, and perching ducks, based on their characteristic behaviours. The mallard, a typical dabbling duck (any of about 38 species of Anas and about 5 species in other genera), is one of the most popular game birds.