Indoor describes anything that happens or is used inside a building. An indoor pool, indoor shoes, an indoor market, the indoor plant dying quietly on the windowsill.
It is an adjective and it goes in front of its noun. You cannot say the game was indoor — for that you need indoors with the s, an adverb: we stayed indoors, come indoors, it rained so we played indoors. That single letter is the mistake to watch, and outdoor and outdoors follow exactly the same rule. Indoor also marks sport held under a roof, where the records are kept separately: the indoor 200 metres, indoor cycling, indoor bowls. Nothing about the word suggests comfort — an indoor firing range is still a firing range.
1adjectiveindoorHappening, used or existing inside a building.
“The hotel has an indoor pool and a sauna in the basement.”
English used to say within doors for inside the house. Indoor is that phrase worn down, within plus door, and it was first written in 1711. The door stands in for the whole building: cross it and you are in.
From (with)in + door, first attested 1711.
Etymology from Wiktionary, CC BY-SA