  Recent scanning tunneling microscopy (STM) experiments of
Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ have shown evidence of real-space
organization of electronic states at low energies in the pseudogap state. We
argue based on symmetry considerations as well as model calculations that the
experimentally observed modulations are due to a density wave of d-wave
Cooper-pairs without global phase coherence. We show that STM measurements can
distinguish a pair-density-wave from more typical electronic modulations such
as those due to charge density wave ordering or scattering from an onsite
periodic potential.
