Outdoor means happening or used in the open air. A pool with no roof, a coat made for weather.
It goes in front of nouns only, which is worth knowing, because the form that stands alone is outdoors with an s: an outdoor pool, but we ate outdoors. It attaches to activities, clothing and furniture — outdoor pursuits, outdoor gear, outdoor seating. There is also the outdoor type, meaning somebody who would rather be walking than sitting inside. The opposite is indoor, which behaves in exactly the same way.
1adjectiveoutdoorSituated in, or done in, the open air.
“The school has an outdoor swimming pool.”
Middle English
dore
English
door
English, today
outdoor
Built from
From out- + door. door comes from Middle English dore. From Middle English dore, dor, from Old English duru (“door”), dor (“gate”), from Proto-West Germanic dur, from Proto-Germanic durz, from Proto-Indo-European dʰwṓr, from dʰwer- (“doorway, door, gate”). 2
Etymology from Wiktionary, CC BY-SA