A sentient cave was an intelligent lifeform that had the outward appearance of a cave.
Ensign Beckett Mariner claimed to have once been trapped in such a cave for weeks, and described it as a "a dark place that knows things." (LD: "Second Contact")
This lifeform was only mentioned in dialogue.
In Star Trek: The Next Generation - Warped by Lower Decks creator Mike McMahan, Beverly Crusher managed to escape one circa stardate 48156.3, using "highly experimental anti-cave pants." The sentient cave was described as "a menacing antagonistic creature."