Decoherence is a quantum-mechanical process that dynamically describes
the apparent loss of quantum coherence due to coupling of the system un-
der observation to other degrees of freedom, which escape direct observation.
Typical examples are given by scattering processes, in which the off-scattered
particles (and/or radiation) are not detected. In such processes quantum
correlations between the observed system and its environment become delo-
calized in an effectively irreversible manner. Such quantum correlations can
neither be seen by observations on one or the other system alone, nor inter-
preted as statistical correlations between existing (i.e., defined, but possibly
unknown) states of local systems. They truly reflect the non-local nature of
quantum mechanics.