Standing wave

A standing wave is what you get when two travelling waves of equal amplitude run through the same medium in opposite directions. Everywhere they reinforce, you get an antinode that oscillates at double amplitude. Everywhere they cancel, you get a node that never moves. The pattern does not travel — it pulses in place.

Standing waves are the generic response of any bounded medium to excitation. Pluck a string fixed at both ends and the outgoing wave reflects off each end, runs back, and the superposition of the incoming and reflecting waves is a standing wave. Same for sound in pipes, electrons in atoms, light in laser cavities, and water in bathtubs. The boundaries select which standing-wave patterns can fit, and those patterns are the modes.