noun.
An artificial structure or excavation made to resemble a cave or cavern.
The Century Dictionary and Cyclopedia
noun.
A subterranean cavity; a natural cavern, or an ornamented excavation or construction more or less remotely resembling a natural cave, made for shade or recreation.
the GNU version of the Collaborative International Dictionary of English
noun.
A natural covered opening in the earth; a cave; also, an artificial recess, cave, or cavernlike apartment.