What does the term "idle speed" refer to?

The term "idle speed" specifically refers to the engine's RPM (revolutions per minute) when the vehicle is at rest and not accelerating. This is the engine's baseline running speed, allowing it to maintain necessary functions like operating the battery, fuel delivery, and other systems without moving the vehicle. Idle speed is essential for ensuring smooth operation and stability while the vehicle is stationary, often measured in RPM.

In contrast, the other options center around different aspects of vehicle operation. The maximum speed on the highway relates to vehicle performance capability, while acceleration speed involves the engine's response to throttle input. The speed limit is governed by laws and regulations, which are unrelated to the technical operation of the engine at rest. Thus, idle speed accurately defines the specific scenario where the engine runs without motion, making option B the correct choice.