Joint

1 in geology, fractures in rock that may occur as parallel sets or, more commonly, in an irregular and less systematic manner. Where a set of joints can be identified, it can usually be related to tectonic (that is to do with deformation of the Earth) stresses and the geometry of the rock body. There are several types of joint: unloading joints, which are caused by the release of stress on rocks at depth as overlying rocks are removed by erosion; cooling joints, which occur in igneous bodies; and joints that are related to regional deformation (on a wide scale).

2 In biology, the point at which bones and the surrounding tissues meet. They may be fixed joints (as in the cranium), slightly movable (e.g., joints in the spine) or freely movable (e.g., knee, arm). The movable joint types also differ -- ball and socket (hip) or hinge (elbow).

Taken from Dictionary of Science