A joint is a fracture or crack in a rock along which there has been no significant movement parallel to the plane of the fracture. Joints are common in many rock types and often occur in sets or systems. They are important in studying the mechanical behavior of rocks, the movement of groundwater, and the formation of geological structures.
Reference: Pollard, D. D., & Aydin, A. (1988). “Progress in Understanding Jointing Over the Past Century.” Geological Society of America Bulletin, 100(8), 1181-1204.