Phase
Where a wave is in its cycle at a given moment, measured as an angle.
See it in Cinemagraph. Pick the Wave animation and watch the swing travel across the picture.
In plain words
A wave repeats a cycle: up, down and back to the start. Phase says how far through that cycle it is. It is given as an angle, where a full cycle is 360 degrees. Two identical waves can be out of step, one a quarter cycle behind the other. Shifting phase row by row is what makes a ripple travel across a picture.