1. Underground opening in soluble rock similar to a cave. When used as a noun, it refers to large openings, but when used as an adjective it tends to refer to rock texture and so to small openings. However, in some countries (e.g., Russia) cavern refers to small openings in a rock . 2. A synonym of cave with the implication of large size. 3. A system or series of caves or cave chambers. 4. A cave, often used poetically or to connote larger-than-average size . Synonyms: (French.) caverne; (German.) Hohle, Kaverne; (Greek.) speleon; (Italian.) caverna, grotta; (Russian.) kaverna; (Spanish.) caverna, cueva; (Turkish.) kovuk; (Yugoslavian.) kaverna. See also cave.