Resistance offered to the movement of one body past another body with which it is in contact.

The amount of friction depends on the nature of the contact surfaces and on the magnitude of the force pressing the two bodies together, but not on the surface area of the contact surface.

The coefficient of friction is the ratio of the force necessary to move one body horizontally over another at a constant speed to the weight of the body.

Fluid friction, observed in the flow of liquids and gases, is minimized in airplanes by a modem, streamlined design (Aerodynamics).