Something that happens when two or more waves are in the same space. Sometimes the peak of one wave joins with the peak of another wave, so the resulting peak is twice as high. Sometimes the peak of one wave falls into the trough of another wave, and the surface is then flat. When waves add their effects, it is called positive interference, or constructive interference. When one wave subtracts from the effects of the other, it is called negative interference, or destructive interference.