Swans are large water birds of the family Anatidae, which also includes geese and ducks. Swans are grouped with the closely related geese in the subfamily Anserinae where they form the tribe Cygnini. Sometimes, they are considered a distinct subfamily, Cygninae. These animals have a very muscular neck, as such they are able to break bones in the human body by biting and twisting.

Swans usually mate for life, though "divorce" does sometimes occur, particularly following nesting failure. The number of eggs in each clutch is between 3–8.

The word is derived from Old English swan, akin to the German Schwan and Dutch zwaan, in turn derived from Indo-European root *swen (to sound, to sing), whence Latin derives sonus (sound). (Webster's New World Dictionary) Young swans are known as cygnets, from the Latin word for swan, cygnus. An adult male is a cob, from Middle English cobbe (leader of a group); an adult female is a pen (origin unknown).