Optics: Offset of the electric or magnetic field from some predefined "zero" in propagating light. Since both fields are cyclical in nature, the phase will return to zero at the end of each cycle. The "absolute phase" is rarely of interest, in most situations only the "relative phase" is important. (it is also impossible to determine the "absolute phase in most cases anyway.)
For instance many optics situations assume a plane wave which is a optical wavefront or line of constant phase as the starting point. After going through some system (a lens for instance), the wavefront is no longer straight (in the case of the lens it's bowed in the shape of the physical lens because the light is delayed proportional to the time spent in the lens.) The shape of the wavefront tells us how the system will change light going through it.
An explanation of almost all optical devices or systems requires an intuitive understanding of optical phase and it's consequences.