1. A water-filled transparent container, often box-shaped, in which fish and other water animals and plants are kept.
2. A tank, bowl, or other water-filled enclosure in which living fish or other aquatic animals and plants are kept.
3. A building in which fish and other water animals are kept and shown to the public.
4. A place for the public exhibition of live aquatic animals and plants.

An aquarium is also known as a vivarium, and is usually contained in a clear-sided container (typically constructed of glass or high-strength plastic) in which water-dwelling plants and animals (usually fish, and sometimes invertebrates, as well as amphibians, marine mammals, and reptiles) are kept in captivity, often for public display; or it is an establishment featuring such displays.

A wide variety of aquaria are now kept by hobbyists, ranging from a simple bowl housing a single fish to complex ecosystems with carefully engineered support systems. Aquaria are usually classified as containing fresh or salt water, at tropical or cold water temperatures.