1. A residual portion of the roof of a subsurface karst cavity which has not collapsed. Such a natural arch may occur as a surface topographic feature, or as a part of a cave system [20]. 2. A rock arch or very short natural tunnel; contrasted with natural bridge, which spans a ravine or valley [10]. Synonyms: (French.) arche naturelle; (German.) naturlisches Gewolbe, Naturbrucke, Felsfenster, Felsbrucke; (Greek.) physike apsitha; (Italian.) arco naturale; (Russian.) estestvennaya arka; (Spanish.) arco natural; (Turkish.) dogal kemer; (Yugoslavian.) prirodni svod, luk, naravni obok. See also natural bridge.