either of two crocodilians related to the tropical American caimans (family Alligatoridae). Alligators, like other crocodilians, are large animals with powerful tails that are used both in defense and in swimming. Their eyes, ears, and nostrils are placed on top of their long head and project slightly above the water when the reptiles float at the surface, as they often do. Alligators can be differentiated from crocodiles (crocodile) by the form of their jaw and teeth. Alligators possess a broad, U-shaped snout and have an “overbite”—that is, all the teeth of the lower jaw fit within (lingual to) the teeth of the upper jaw. The large fourth tooth on each side of the alligator's lower jaw fits into a socket in the upper jaw. Usually, no lower teeth are visible when the mouth is closed. In contrast, crocodiles have a narrow, V-shaped snout, and the large fourth tooth on each side of the crocodile's lower jaw projects outside the snout when the mouth is closed. Alligators are carnivorous and live along the edges of permanent bodies of water, such as lakes, swamps, and rivers. They commonly dig burrows in which they rest and avoid weather extremes.

The Chinese alligator (A. sinensis) is a much smaller, little-known reptile found in the Yangtze River region of China. It is similar to the larger form but attains a maximum length of about 2.1 metres (7 feet)—although usually to 1.5 metres—and is blackish with faint yellowish markings. It is considered endangered by the International Union for Conservation of Nature and Natural Resources (IUCN).