located, suited for, or taking place within a building
"indoor activities for a rainy day"
"an indoor pool"
Situated in, or designed to be used in, or carried on within, the interior of a building.
"The new indoor playground is heated and air-conditioned so children can play comfortably even during a snowstorm."
In plain English: Indoor means something that happens inside a building instead of outside.
"The game was moved indoors because it started to rain."
Usage: Use indoor to describe anything located or intended for use inside a building, such as an indoor pool or indoor shoes. Do not confuse it with outdoor when referring to activities or items meant for the open air.
The word indoor is a combination of within and door that was first recorded in English in 1711. It literally means being inside the doorway or entrance.