What is an incident wave?

An incident wave emanates from a source of wave production. If there is a boundary from which this wave is reflecting, the returning wave is known as the reflected wave.

The interaction between incident waves and reflected waves can form standing waves that double the amplitude of either wave separately. Whether the two waves form constructively to form such standing waves depends on the phase change that the incident wave experienced at the boundary. If the phase was a 180 degree change, then the incident and wave and reflected wave interfere destructively and the amplitude of the combination is zero.