1. The rubbing of one body (or thing) against another; attrition.
2. The resistance which any body meets with in moving over another body.
3. The rubbing of two objects against each other when one or both are moving.
4. In physics, the resistance encountered by an object moving relative to another object with which it is in contact.
5. In medicine, deliberate rubbing of a body part as a way of stimulating blood circulation, warming, or relieving pain.
6. Disagreement or conflict, stopping short of violence, between individuals, groups, or nations with differing aims or views.

In physics, friction is a force that opposes the relative motion of two material surfaces that are in contact with one another; the direcition of the force on each body is opposite to the direction of its motion relative to the other body.

The thin layer of air adjacent to the Earth's surface; surface layer: The atmospheric boundary layer is usually considered to be less than 300 feet (91 meters) high.

In the Earth's atmosphere, the planetary boundary layer is the air layer near the ground affected by diurnal heat, moisture, or momentum transfer to or from the surface.

The thin layer of air adjacent to the Earth's surface; ground layer: The atmospheric boundary layer extends up to the so-called anemometer level (the base of the Ekman layer [thin top layer of the sea]). Within this layer the wind distribution is determined largely by the vertical temperature gradient and the nature and contours of the underlying surface, and shearing stresses are approximately constant.

The dissipation of energy which occurs when a particle in a vibrating system is resisted by a force whose magnitude is a constant independent of displacement and velocity, and whose direction is opposite to the direction of the velocity of the particle.

In mechanics, a frictional force between dry stationary surfaces which is proportional to the normal force.

The thin layer of the atmosphere adjacent to the earth's surface.