  Entropy and temperature of a system in a coherent state are naturally defined
on a base of a density matrix of the system. As an example, entropy and
temperature are evaluated for coherent states of a harmonic oscillator and
quantum field described by the Klein--Gordon--Fock equation with a source term.
It is shown, in particular, that the temperature of the coherent oscillator in
a ground state coincides with the effective temperature of a harmonic
oscillator being in contact with a heat bath (Bloch formula) when the bath
temperature tends to zero. The Bekenstein--Hawking entropy of a black hole can
also be interpreted as an entropy of coherent states of a physical vacuum in
the vicinity of a horizon surface.
