1. A fracture in the earth's crust, across which relative rock movement has taken place, or continues to take place. Fault planes commonly guide vertical or sub-vertical shafts in caves, as well as guiding sub-horizontal or oblique passages within the confines of the fault plane [9]. 2. A fracture or fracture zone along which there has been displacement of the two sides relative to one another parallel to the fracture [6]. This displacement may be of a few centimeters or many kilometers. See also joint fault set; joint fault system.