  An expression for the photon condensate in quantum electrodynamics is
presented and generalized to deduce a simple relation between the gluon
condensate and the running coupling constant of quantum chromodynamics (QCD).
Ambiguities in defining the condensates are discussed. The values of the gluon
condensate from some Ans\"{a}tze for the running coupling in the literature are
compared with the value determined from QCD sum rules.
