Indoor means inside a building rather than outside in the open air. When you play indoor soccer, you're in a gymnasium instead of on an outdoor field. Indoor plants grow in pots inside your house, while outdoor plants grow in gardens or forests.

The word describes anything that happens or belongs inside: indoor pools stay warm all winter, indoor voices stay quieter than playground voices, and indoor shoes don't track mud everywhere. Some activities work perfectly well indoors: reading, cooking, or playing board games. Others, like flying kites or having a bonfire, really need to happen outdoors.

People sometimes use indoors (with an “s”) as an adverb meaning “inside”: “Come indoors before it starts raining” or “Let's eat indoors tonight.” Both forms mean the same basic thing; they're just used differently in sentences. The opposite of indoor is outdoor, and together these words help us describe where things happen or belong.