Duck is the common name for a large number of species in the waterfowl family Anatidae, which also includes swans and geese. Ducks are divided among several subfamilies in the family Anatidae; they do not represent a monophyletic group (the group of all descendants of a single common ancestral species) but a form taxon, since swans and geese are not considered ducks. Ducks are mostly aquatic birds, mostly smaller than the swans and geese, and may be found in both fresh water and sea water. The word duck comes from Old English *dūce "diver", a derivative of the verb *dūcan "to duck, bend down low as if to get under something, or dive", because of the way many species in the dabbling duck group feed by upending; compare with Dutch duiken and German tauchen "to dive".

This word replaced Old English ened/ænid "duck", possibly to avoid confusion with other Old English words, like ende "end" with similar forms. Other Germanic languages still have similar words for "duck", for example, Dutch eend "duck" and German Ente "duck". The word ened/ænid was inherited from Proto-Indo-European; compare: Latin anas "duck", Lithuanian ántis "duck", Ancient Greek nēssa/nētta (νῆσσα, νῆττα) "duck", and Sanskrit ātí "water bird", among others.

A duckling is a young duck in downy plumage or baby duck.

A male duck is called a drake and the female is called a duck, or in ornithology a hen.

The overall body plan of ducks is elongated and broad, and the ducks are also relatively long-necked, albeit not as long-necked as the geese and swans. The body shape of diving ducks varies somewhat from this in being more rounded. The bill is usually broad and contains serrated lamellae, which are particularly well defined in the filter-feeding species. In the case of some fishing species the bill is long and strongly serrated. The scaled legs are strong and well developed, and generally set far back on the body, more so in the highly aquatic species. The wings are very strong and are generally short and pointed, and the flight of ducks requires fast continuous strokes, requiring in turn strong wing muscles. Three species of steamer duck are almost flightless, however. Many species of duck are temporarily flightless while moulting; they seek out protected habitat with good food supplies during this period. This moult typically precedes migration.

The drakes of northern species often have extravagant plumage, but that is moulted in summer to give a more female-like appearance, the "eclipse" plumage. Southern resident species typically show less sexual dimorphism, although there are exceptions like the paradise shelduck of New Zealand which is both strikingly sexually dimorphic and where the female's plumage is brighter than that of the male. The plumage of juvenile birds generally resembles that of the female. Over the course of evolution, female ducks have evolved to have a corkscrew shaped vagina to prevent rape.

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