What is a stationary wave?

A stationary wave forms when there are two waves of equal frequency (or wavelength) travelling in opposite directions, passing through one another. Interference occurs, with constructive interference at antinodes and destructive interference at nodes. That is to say, there is maximum displacement at antinodes and no displacement at nodes. Particles either side of the nodes are in antiphase (between two nodes in phase). Amplitude may vary between nodes.