A joint (from the Greek "διά" dia, through, and klasis, break) is a naturally occurring break (fracture) in a rock that has very little movement between both sides of the fracture.[1]​[2]​ Joints do not usually have mineral fillings.[3]​ Most joints reflect a small extension movement.[1]​[4]​.

Faults differ from joints in that the former exhibit visible or measurable movement between both sides of the fracture.[5][6][7][8]​.

Joints are usually related to the geometry of the rock masses where they occur as well as to the history of tectonic stress of these.[9]​ In some cases it has been noted that joints are related to folding, with joints being both parallel and oblique to the axial plane.[9]​[10]​[11]​ A common trend between parallel joints is that the longer the joints, the greater the distance between them. others.[12]​.

Exfoliation joints are formed by depressurization of the rock when close to a free surface.[13]​ These joints tend to be parallel to said surface.[9]​[13]​ In sedimentary rocks they tend to be parallel to the stratification.[9]​[10]​ Exfoliation joints can form in rocks at depth when they lose their overburden during exhumation "Exhumation (geology)") of the rock.[9]​ These joints have a notable development in massive igneous rocks,[9][10]​ for example in inselbergs.[14]​.

Some joints are formed by cooling of the rock, for example those known as columnar disjunction.[10]​.

The orientation of a joint, like that of other geological structures, is described by two parameters:

Joints are commonly represented geometrically as planes.[1][4]​.

Joints do not usually appear isolated, but rather associated with faults and folds. When, as is usually the case, there are two or more sets of joints, we speak of a joint system or "joint system". The simplest are:

In order to discriminate between compression and distention joints, the main axes of local or regional deformation must be studied, since the joints themselves do not provide sufficient information (striations or displacement). In the case of extension joints, the direction of the most noticeable family is usually perpendicular to the direction of extension and in compression joints the direction of the bisector of the acute angle of the joint intersection.