1. A water-filled transparent container, often box-shaped, in which fish and other aquatic or water animals and plants are kept: "The teacher had an aquarium in the classroom for the students to observe and learn about a variety of fresh water flora and fauna."

"As a child, Adam was given an aquarium which contained several fish to keep as a hobby."

2. A building in which fish and other water animals are kept and shown to the public: "There were several aquaria in the natural history museum for both fresh and salt water fish."

"During the summer, many tourists visit the outdoor aquaria near the seashore."

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.