  This paper introduces cosmoDC2, a large synthetic galaxy catalog designed to
support precision dark energy science with the Large Synoptic Survey Telescope
(LSST). CosmoDC2 is the starting point for the second data challenge (DC2)
carried out by the LSST Dark Energy Science Collaboration (LSST DESC). The
catalog is based on a trillion-particle, 4.225 Gpc^3 box cosmological N-body
simulation, the `Outer Rim' run. It covers 440 deg^2 of sky area to a redshift
of z=3 and is complete to a magnitude depth of 28 in the r-band. Each galaxy is
characterized by a multitude of properties including stellar mass, morphology,
spectral energy distributions, broadband filter magnitudes, host halo
information and weak lensing shear. The size and complexity of cosmoDC2
requires an efficient catalog generation methodology; our approach is based on
a new hybrid technique that combines data-driven empirical approaches with
semi-analytic galaxy modeling. A wide range of observation-based validation
tests has been implemented to ensure that cosmoDC2 enables the science goals of
the planned LSST DESC DC2 analyses. This paper also represents the official
release of the cosmoDC2 data set, including an efficient reader that
facilitates interaction with the data.
