In mechanical engineering, what does the term 'friction' refer to?

Friction is defined as the resistance that occurs when two surfaces slide or attempt to slide against each other. This resistance can significantly impact the performance of mechanical systems, influencing factors like efficiency, wear, and heat generation. In engineering applications, understanding friction is crucial for the design and operation of machines, as it affects everything from the motion of components to the energy consumption of the system.

The other options provided do not correctly define friction. While propulsion relates to force, it doesn't encompass the resistance aspect that characterizes friction. The weight of an object is simply a measure of gravitational force acting on it, which is unrelated to the frictional forces at play. Velocity, while a fundamental aspect of motion, does not pertain to the concept of resistance between surfaces.