  We define an effective temperature and study its properties for a class of
out-of-equilibrium steady states in a heat bath. Our analysis is based on the
anti-de Sitter spacetime/conformal field theory (AdS/CFT) correspondence, and
examples include systems driven by applied electric fields and branes dragged
in plasmas. We found that the effective temperature can be lower than that of
the heat bath and that the out-of-equilibrium noise can be smaller than that in
equilibrium. We show that a generalization of the fluctuation-dissipation
relation holds for the effective temperature. In particular, we generalize the
Johnson-Nyquist relation for large electric field.
