In physics, a standing wave – also known as a stationary wave – is a wave that remains in a constant position.
Two opposing waves combine to form a standing wave.
This phenomenon can occur because the medium is moving in the
opposite direction to the wave, or it can arise in a stationary medium
as a result of interference between two waves traveling in opposite directions. In the second case, for waves of equal amplitude traveling in opposing directions, there is on average no net propagation of energy.
In a resonator, standing waves occur during the phenomenon known as resonance.