| JOINT | A planar or gently curving crack separating two parts of once continuous rock, without relative movement along its plane. Speleology term. Ref JJ |
| JOINT PLANE | The plane in which a joint has formed. Speleology term. Ref MM |
| JOINT PLANE CAVE | A cavity developed along a joint, which is often steeply dipping, and thus high in relation to width. Cave - Kinds of. Ref JJ |
| JUG HANDLE | A small loop of rock shaped like a handle, used as an anchor point or hand hold to aid in climbing. Climbing and SRT. Ref MM |
| JUMAR | An ascender with a simple finger operated safety catch, a handle and several attachment points. Climbing and SRT. Ref JJ |