Definition: From quantum physics, coherence is a property exhibited by matter and energy when it remains unmeasured and is thus able to exist in more than one state simultaneously (superposition).
The "wave function" is a math formula that describes all possible futures and possible pasts for a system. Until a measurement is made the system is coherent and all possibilites are both true and false. Once a measurement is made, the wave function collapses and the state is determined. Any interaction can be considered a measurement, which is why coherence is such a delicate property to maintain.
