A terrarium is a container which is designed to hold small plants and animals in controlled conditions. The closed nature of a terrarium creates an environment which is easy to control, allowing people to simulate everything from the desert to the rainforest. Terrariums are often used by scientific researchers to conduct studies, and people also keep them as hobbies to display various plants and creatures of interest. Terrariums can be purchased from companies which specialize in them, and they can also be made by hand; people who want especially large or custom-fitted terrariums often end up making their own.

The defining feature of a terrarium is that it is an enclosed replica of a natural environment which is in contact with the earth, so some sort of soil, sand, or rock must be present in a terrarium. Typically, the container is clear, allowing an unobstructed view of the contents, although a terrarium can also be built from wood and other materials with sliding panels to allow a view.

When the top of a terrarium is closed, it tends to be warmer and more humid, while open-topped terrariums are cooler. If animals are kept inside, screens may be used to allow ventilation without permitting escape; rare plants may also be protected with screens on the off chance that insect predators take an interest in the plant life.

Lighting in a terrarium can be controlled with the use of internal and external fixtures. Automated lighting systems can even be used to simulate natural conditions, with a set number of hours of daylight followed by night. Temperature is controlled with the assistance of heat lamps and heaters inside the terrarium, such as heated rocks, with users keeping a thermometer inside to track the conditions. Controlling humidity is also important, as is the regulation of ventilation.

People who keep insects and small reptiles usually use terrariums for their charges, sometimes simulating the natural environment of their pets to make them feel more comfortable. A terrarium may be small enough to fit on a countertop or a desk in a classroom, or it may be much larger; entire rooms can be turned into terrariums, for example, for the purpose of studying large plants and animals. To prevent escape, a double door system is often used, so that the outside door never opens directly onto the inside.

There are also some specialized types of terrarium. An insectarium, for example, is a terrarium designed for keeping insects, while a lepidoptery is a facility for raising and studying butterflies. A paludarium includes a water features, such as a flowing stream, and may represent a cross between an aquarium and a terrarium.