  We recently identified a population of low surface brightness objects in the
field of the z=0.023 Coma cluster, using the Dragonfly Telephoto Array. Here we
present Keck spectroscopy of one of the largest of these "ultra-diffuse
galaxies" (UDGs), confirming that it is a member of the cluster. The galaxy has
prominent absorption features, including the Ca II H+K lines and the G-band,
and no detected emission lines. Its radial velocity of cz=6280 +- 120 km/s is
within the 1 sigma velocity dispersion of the Coma cluster. The galaxy has an
effective radius of 4.3 +- 0.3 kpc and a Sersic index of 0.89 +- 0.06, as
measured from Keck imaging. We find no indications of tidal tails or other
distortions, at least out to a radius of ~2 r_e. We show that UDGs are located
in a previously sparsely populated region of the size - magnitude plane of
quiescent stellar systems, as they are ~6 magnitudes fainter than normal
early-type galaxies of the same size. It appears that the luminosity
distribution of large quiescent galaxies is not continuous, although this could
largely be due to selection effects. Dynamical measurements are needed to
determine whether the dark matter halos of UDGs are similar to those of
galaxies with the same luminosity or to those of galaxies with the same size.
