In ABS, what does the term "wheel slip" refer to?

The term "wheel slip" in Anti-lock Braking Systems (ABS) refers specifically to the situation when the wheels are spinning faster than the actual speed of the vehicle's movement. This typically happens during acceleration or braking when the wheels lose traction with the road surface. As the vehicle attempts to accelerate or brake, if the wheels begin to spin without effectively moving the vehicle forward (due to poor grip), this results in wheel slip.

Understanding wheel slip is crucial for the functionality of ABS, which is designed to monitor wheel speed and prevent slip by modulating brake pressure. This ensures that the wheels maintain some level of grip with the road surface, allowing for controlled braking and preventing skidding.

The other options reflect different scenarios not representative of wheel slip. When wheels do not rotate at all, that suggests a complete loss of motion rather than slip. If wheels are spinning at the same rate as the vehicle, that indicates effective traction, and thus no slip is occurring. Locked-up wheels during braking lead to a loss of control and can cause skidding, which is also not synonymous with wheel slip.