A geological formation consisting of an underground enclosure with access from the surface of the ground or from the sea WordNet 3.0 & Open English WordNet

A cave or cavern is a natural void in the ground, specifically a space large enough for a human to enter. Wikipedia

Subterranean chamber. Wikipedia Disambiguation

Natural underground space large enough for a human to enter Wikidata

1) An underground hollow with access from the ground surface or from the sea, often found in limestone areas and on rocky coastlines. 2) A natural cavity, chamber or recess which leads beneath the surface of the earth, generally in a horizontal or obliquely inclined direction. It may be in the form of a passage or a gallery, its shape depending in part on the joint pattern or structure of the rock and partly on the type of process involved in its excavation. Thus, caves worn by subterranean rivers may be different in character from, and of considerably greater extent than, a sea-cave eroded by marine waves. 3) A natural underground open space, generally with a connection to the surface and large enough for a person to enter. The most common type of cave is formed in a limestone by dissolution. OmegaWiki

A naturally-occurring cavity in bedrock which is large enough to be entered by an adult. Wiktionary

Caving: naturally-occurring cavity large enough to be entered by an adult. Wiktionary (translation)