What condition is described as water hammer in plumbing systems?

Water hammer is a condition that occurs in plumbing systems when there is a sudden change in water flow, typically caused by the rapid opening or closing of a valve. This rapid change creates a pressure surge within the pipes, which can lead to a loud banging noise and potentially damaging vibrations. The phenomenon is attributed to the inertia of the moving water and the pressure wave generated as the flow is abruptly stopped or redirected.

When a valve is operated quickly, the water has little time to adjust to the sudden change in pressure, resulting in this surge. This is why the correct answer focuses on a pressure surge caused by quick valve operation, capturing the essence of how water hammer develops in a plumbing context.