  We describe a standard star catalog constructed using multiple SDSS
photometric observations (at least four per band, with a median of ten) in the
$ugriz$ system. The catalog includes 1.01 million non-variable unresolved
objects from the equatorial stripe 82 ($|\delta_{J2000}|<$ 1.266$^\circ$) in
the RA range 20h 34m to 4h 00m, and with the corresponding $r$ band
(approximately Johnson V band) magnitudes in the range 14--22. The
distributions of measurements for individual sources demonstrate that the
photometric pipeline correctly estimates random photometric errors, which are
below 0.01 mag for stars brighter than (19.5, 20.5, 20.5, 20, 18.5) in $ugriz$,
respectively (about twice as good as for individual SDSS runs). Several
independent tests of the internal consistency suggest that the spatial
variation of photometric zeropoints is not larger than $\sim$0.01 mag (rms). In
addition to being the largest available dataset with optical photometry
internally consistent at the $\sim$1% level, this catalog provides practical
definition of the SDSS photometric system. Using this catalog, we show that
photometric zeropoints for SDSS observing runs can be calibrated within nominal
uncertainty of 2% even for data obtained through 1 mag thick clouds, and
demonstrate the existence of He and H white dwarf sequences using photometric
data alone. Based on the properties of this catalog, we conclude that upcoming
large-scale optical surveys such as the Large Synoptic Survey Telescope will be
capable of delivering robust 1% photometry for billions of sources.
