From King Dictionary of Contemporary Englishindoorin‧door /ˈɪndɔː $ -ɔːr/ ●●○ adjective [only before noun] IN/INSIDEused or happening inside a building OPP outdoor an indoor swimming pool the world indoor athletics championship Too much central heating can harm indoor plants.USAGE: Indoor, indoorsIndoor is an adjective and indoors is an adverb.You say: indoor sports Don't say: indoors sportsYou say: I stayed indoors. Don't say: I stayed indoor.
Examples from the Corpus
indoorindoor lightingTraditionally, markets were held outdoors, but many cities in Britain now have indoor markets.indoor soccerThe hotel has a heated indoor swimming pool.This year, the world indoor tennis championship will take place in Paris.