Hi iufan24, There is almost no difference between '실외' and '야외'. Both are translated as 'the outside' or 'the outdoor'. Ex. 우리는 시청 앞에 있는 실외 (or 야외) 수영장에 갔다. = We went the outdoor swimming pool in front of the city hall. 실내보다 실외가 운동하기 편하다. = The outside is more comfortable to work out than the inside. Hope it helps. )