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The term that describes the amount of friction between two moving faces in contact with each other, expressed as a ratio, is the coefficient of friction. This coefficient is a dimensionless number that quantifies the frictional force between two surfaces. It is calculated by taking the ratio of the force of friction (the force required to move one surface over the other) to the normal force (the force perpendicular to the surfaces in contact).

The coefficient of friction can vary depending on the materials involved and their surface conditions, such as whether they are dry, lubricated, or worn. Understanding this concept is crucial in brake system design and performance, as it directly impacts the effectiveness of braking systems, vehicle control, and overall safety.

In contrast, other terms listed do not accurately describe this concept. For example, while "friction" refers to the actual resisting force itself, it does not express it as a ratio. Similarly, "the coefficient of heat" and "heat ratio" are not terms used to describe frictional interaction between surfaces. Thus, the coefficient of friction is the appropriate term for this context.