We say a message is incoherent when we can't make it out, or when it doesn't make sense. A scribbled note, a drunken argument or a conversation taking place five tables down in a crowded cafe might all be incoherent. In general, "coherent" means the opposite—consistent, connected, clear.
In science, the word coherence takes on more specific, mathematical definitions, but they all get at a similar concept: Something is coherent if it can be understood, if it forms a unified whole and if those first two qualities persist.
Scientists originally developed the concept of coherence to understand and describe the wave-like behavior of light. Since then, the concept has been generalized to other systems involving waves, such as acoustic, electronic and quantum mechanical systems.
"Coherence is a measure of how well certain systems will maintain their relationships with each other and how well we are able to predict the evolution of those systems," said Martin Holt, a scientist at the U.S. Department of Energy's (DOE) Argonne National Laboratory and a member of Q-NEXT, a DOE National Quantum Information Science Research Center led by Argonne. "Understanding and controlling coherence in quantum technologies is crucial because the relationships involved need to be very long-lived and well-understood."
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