  We discuss the class of superconductors which have pairing correlations which
are odd in frequency, as introduced originally by Berezinskii and more recently
by Balatsky and Abrahams. As follows from the equations of motion, a natural
definition of the thermodynamic order parameter of the odd-pairing state is the
expectation value of a composite operator which couples a Cooper pair to a spin
or charge fluctuation. We use a model pairing hamiltonian to describe
properties of the odd-pairing composite-operator condensate. We show that the
superfluid stiffness is positive, we discuss superconductive tunneling with an
ordinary superconductor and we derive other thermodynamic and transport
properties.
