Planing (n.) refers to the forces developed by a moving fluid such as water, on an solid object at an interface between that fluid and one of less density, such as air.
The role of planing in surfing is unique in that forces of planing commonly provide the means of propulsion. This is in contrast to the more common usage of the term in which the term is a reference to the way in which a moving watercraft, which is being propelled by an external power source such as an outboard motor, rises out of the water. Hence, the common reference to planing as a dynamic buoyancy, or lift. Both of which are applicable in surfing, but because of the application - the flow of water up/forward on the wave face being the power source - the forces of planing also provide components which allow the surfcraft to move in forward and transverse directions.