洞穴 (dòngxué) is a compound noun meaning 'cave'—a natural underground hollow or passage formed in rock, typically by water erosion over long periods. The first character, 洞 (dòng), means 'hole' or 'cavity', suggesting openness and depth; the second, 穴 (xué), means 'cavity', 'burrow', or 'den', often implying a sheltered, enclosed space. Together, they emphasize a geological feature that is both spacious and secluded—distinct from man-made holes (e.g., 隧道 suìdào, 'tunnel') or small crevices (e.g., 缝隙 fèngxì, 'crack').

This word is used primarily in scientific, geographical, or descriptive contexts—such as cave exploration (洞穴探险 dòngxué tànxǐan), karst landscapes, or paleontological sites. It carries a neutral, formal register and appears frequently in textbooks, documentaries, and travel writing about natural landmarks. While it can occasionally refer metaphorically to hidden or mysterious places, its core meaning remains strictly geological and physical.