A terrarium is a clear glass or plastic container with soil and small plants growing inside it, kept indoors as decoration. Usually it is closed or nearly closed, so water rises off the plants, gathers on the glass, and drips back down — a tiny world that mostly waters itself. People keep them on desks and shelves; making them is a popular hobby, and shops sell little tools with long handles for reaching inside. The word is also used for a glass tank where someone keeps a small land animal — a lizard, a snake, a spider — the dry cousin of a fish tank. Both uses are normal; which one a listener pictures depends on the room. It is a slightly bookish word: everyday speakers may just say 'a glass jar with plants in it', and the word turns up more in shops, catalogues, and hobby videos than in casual talk.