Friction's the force resisting the relative lateral (tangential) motion of solid surfaces, fluid layers, or material elements in contact. It's usually subdivided into several varieties: Dry friction resists relative lateral motion of two solid surfaces in contact. Dry friction's also subdivided into static friction between non-moving surfaces, and kinetic friction (sometimes called sliding friction or dynamic friction) between moving surfaces. Lubricated friction or fluid friction resists re… (More on Friction)