Definition:Friction

Definition
Friction is a force whose tendency is to reduce the velocity of a body relative to its surroundings.

It is caused by interaction of the matter in the body with that of the environment in which it is traveling. Friction is (at a fundamental level) an electromagnetic force.

That is, bits of the medium in or on which the body is traveling get in the way (on a molecular level) of bits of the body.

The less friction, the less the reduction in velocity.

If there were no friction, then motion would continue for ever.

Linguistic Note
The word friction comes from the Latin word for rub.