The ratio of the weight of an object being moved along a surface and the force that maintains contact between the object and the surface WordNet 3.0 & Open English WordNet

The ratio between (1) the magnitude of the force of friction which a surface produces on an object (moving along the surface or being pressed statically against it) and (2) the magnitude of the normal force which is produced by the surface on the object and which is perpendicular to that surface. Wiktionary

Ratio. Wiktionary (translation)