From the Latin co- (together) and haerere (to stick), translating as sticking together. 1. The property of a set of electromagnetic waves, consistently similar in terms of a feature such as polarization or phase. Signals consistently synchronized in phase are characterized by oscillations that maintain a fixed relationship, with the sine waves rising and falling in unison. See also phase, polarization, and sine wave. 2. The property of a light source that fires within a narrow range of wavelengths, ideally only one, so that all photons act identically. See also wavelength and window.