The act of rubbing the surface of one body against that of another; attrition; in hygiene, the act of rubbing the body with the hand, with flannel, or with a brush etc., to excite the skin to healthy action.

The resistance which a body meets with from the surface on which it moves. It may be resistance to sliding motion, or to rolling motion.

A clashing between two persons or parties in opinions or work; a disagreement tending to prevent or retard progress.

Features of war that resist all action, make the simple difficult, and the difficult seemingly impossible. Friction may be mental (such as indecision) or physical (such as enemy fire). Friction may be imposed by enemy action or a variety of other physical and human factors. Fear is a key factor in the appearance of friction in military operations.