Phase refers to the argument of a sine or cosine function in a wave equation and depends on position and time, while the phase constant is determined by initial conditions. The phase difference between two points on a wave is the difference between their phases and is equal to the product of the wavenumber and their distance separation. If this distance separation is an integer multiple of the wavelength, the phases are in phase, but if it is an odd half-integer multiple, the phases are out of phase.