A terrarium (plural: terraria or terrariums) is usually a sealable glass container containing soil and plants that can be opened for maintenance to access the plants inside; however, terraria can also be open to the atmosphere. Terraria are often kept as ornamental items.

A closed terrarium's transparent walls allow heat and light to enter, creating a unique environment for plant growth. Heat entering the sealed container allows the creation of a small water cycle due to evaporating moisture from the soil and plants. The water vapor then condenses onto the walls of the container, eventually falling back onto the plants and soil below. Light passing through the transparent walls, allowing photosynthesis, with the constant water supply provide an ideal environment for plants. Open terraria are not sealed and are better suited to plants requiring a more arid environment.