In physics, a measure of the correlation between all of the physical properties of waves, e.g., amplitude, phase, and frequency. It is pointed out by Stephen Elliott that coherence results in the collision of two perfectly coherent waves that one wave negates the other:
http://www.bmedreport.com/archives/18731
The issue of harmony can be resolved by recalling some mathematics. A pair of sinusoidal waveforms are in harmony, provided the one waveform can be superimposed on the other one. Generally, waveforms (regardless of the shape) are in harmony, provided the waveforms can be perfectly superimposed on each other. There is approximate harmony between waveforms, provided one waveform can almost be superimposed on the other.