Usually, the
vector component of the
contact force between two surfaces which
lies in the plane of the surfaces, while the surfaces are moving
relative to each other. The force's
magnitude is
proportional to the magnitude of the
normal force.
A common myth perpetrated by high school physics teachers is that the proportionality constant, the coefficient of kinetic friction is in fact a constant for a given pair of materials. This approximation does make the math much easier to work with, and is reasonably accurate for low speeds. However, if the relative velocity of the two surfaces gets above some speed (which varies depending on what the surfaces are), a better approximation is that the Kinetic Friction Force is proportional to the Normal Force AND to the Velocity.