The coefficient of friction is defined as:

The coefficient of friction is defined as the ratio of the force required to move a body to the normal reaction force acting on the surfaces in contact. This definition is fundamental in the study of friction, as it quantifies the relationship between the frictional force that must be overcome to initiate or sustain movement and the force pressing the two surfaces together (the normal force).

In practical terms, the coefficient of friction varies depending on the materials in contact and the condition of those surfaces (e.g., lubricated or dry). A higher coefficient indicates that more force is needed to overcome the friction between the surfaces. This ratio is dimensionless, making it a valuable property for engineers and scientists to assess performance in applications such as machinery, vehicle tires, and construction materials. Understanding this concept is essential for calculating forces and making decisions related to safety and efficiency in energy systems and equipment.

The other options do relate to forces and concepts involved in friction but do not define the coefficient of friction accurately. The downward force and the normal reaction force are components of the forces at play, while the force parallel to the surface only describes a specific circumstance of motion but does not convey the ratio that defines the coefficient itself.