What is meant by valve hunting?

Ideally, the valve should settle into a single position once setpoint is reached. A hunting valve over-controls, or goes too far, then reverses in an attempt to the correct overshoot, but goes too far, reverses direction again, and constantly repeats this cycle.

A hunting valve results in poor control, and prematurely wears out the valve's mechanical parts.

Hunting is usually indicative of loop gain too high (PID gain + process gain), or a valve that is oversized for the intended flow, or noisy input signal, or a PID integral term too small.