  We present four improved empirical relationships useful for estimating the
central black hole mass in nearby AGNs and distant luminous quasars alike using
either optical or UV single-epoch spectroscopy. These mass-scaling
relationships between line widths and luminosity are based on recently improved
empirical relationships between the broad-line region size and luminosities in
various energy bands and are calibrated to the improved mass measurements of
nearby AGNs based on emission-line reverberation mapping. The mass-scaling
relationship based on the Hbeta line luminosity allows mass estimates for
low-redshift sources with strong contamination of the optical continuum
luminosity by stellar or non-thermal emission, while that based on the C IV
lambda 1549 line dispersion allows mass estimates in cases where only the line
dispersion (as opposed to the FWHM) can be reliably determined. We estimate
that the absolute uncertainties in masses given by these mass-scaling
relationships are typically around a factor of 4. We include in an Appendix
mass estimates for all the Bright Quasar Survey (PG) quasars for which direct
reverberation-based mass measurements are not available.
