1. The property of a single wave with a phase that is a continuous, linear function of position at a given time. A stable local oscillator produces a coherent wave. 2. The property of two or more waves that are in phase both temporally and spatially. Waves are coherent if they have the same wavelength and a fixed phase relationship with each other. When the phase relationships are not fixed, the waves are said to be partially coherent or incoherent. 3. The correlation coefficient between electromagnetic fields at points separated in space and time, sometimes called degree of coherence. So defined, the coherence equals unity for waves that are perfectly coherent and is less than unity for partially coherent waves. 4. As used by Sir Gilbert Walker (1932), the statistical persistence exhibited by successive daily values of atmospheric pressure at any one location.