noun
A natural or semi-natural cave-like space, often with a sense of shelter or seclusion. Can also refer to a cave used as a dwelling or hideout.
海岸の岩屋で潮が引くのを待った。
We waited for the tide to go out in the seaside cavern.
その山には古い岩屋がいくつもある。
There are several old grottoes in that mountain.
Compound of 岩 (iwa, 'rock') and 屋 (ya, 'house' or 'shop'), literally 'rock house'. The spelling 窟 is a single kanji with the same meaning, likely an older or more literary form.