[@Sch], which has lately become the standard reference on the subject, a state is said to be in a *coherent superposition* when each of the component states $|\psi_n\rangle$ is simultaneously present in the state $|\Psi\rangle = \sum_n c_n|\psi_n\rangle$. It is hard to see what the adjective of “coherent” adds to the standard notion of a superposition and what is the part that gets lost through the purely unitary interaction at play. It could be that what is really meant is that when the state of a composite systems is entangled, then each component does not possess a well-defined state. However, if that is all there is to the loss of coherence, then one must conclude that any entangled system, such as an EPR pair, is not coherent. In fact, below we will describe in detail what is (probably) meant by the language in the quote above. All we want to point out here is that such expressions are often repeated without an adequate understanding of the fact that they are only figures of speech to be unpacked in formal terms, and that such unpacking is much more subtle and complex than is usually thought.

This way of talking about decoherence 