The document discusses phase and phase difference in harmonic waves. It defines phase as the angular argument of a wave's function, measured in radians. Phase difference is the difference in phases between two points on a wave, with a unit of Δφ. The phase difference equation is provided. Two points are in phase if they are an integer multiple of wavelengths apart, and out of phase if they are an odd-half integer multiple apart. An example problem calculating phase difference is worked out.