The border between two adjacent lithosphere plates.

An area where two of the plates that make up the surface of the Earth either meet or are pulling away from each other. Where two plates meet, the edge of one plate is usually pulled under the edge of the other in a process known as subduction. The subducted plate may begin to melt as the edge comes into contact with the molten layers underneath it, and this new molten material may rise to the surface of the plate as a line of volcanoes. Where two plates pull apart, a break or weak point in the Earth's crust is created and this becomes plugged with new molten material rising from the molten layers beneath the plates. Plate boundaries are areas of great geological instability and are often rich in earthquakes and volcanic activity.

The edge between adjacent plates, classified by the relative motion taking place: convergent, divergent, or transform.