  We announce the second data release (DR2) of the NOIRLab Source Catalog
(NSC), using 412,116 public images from CTIO-4m+DECam, the KPNO-4m+Mosaic3 and
the Bok-2.3m+90Prime. NSC DR2 contains over 3.9 billion unique objects, 68
billion individual source measurements, covers $\approx$35,000 square degrees
of the sky, has depths of $\approx$23rd magnitude in most broadband filters
with $\approx$1-2% photometric precision, and astrometric accuracy of
$\approx$7 mas. Approximately 1.9 billion objects within $\approx$30,000 square
degrees of sky have photometry in three or more bands. There are several
improvements over NSC DR1. DR2 includes 156,662 (61%) more exposures extending
over 2 more years than in DR1. The southern photometric zeropoints in $griz$
are more accurate by using the Skymapper DR1 and ATLAS-Ref2 catalogs, and
improved extinction corrections were used for high-extinction regions. In
addition, the astrometric accuracy is improved by taking advantage of Gaia DR2
proper motions when calibrating the WCS of individual images. This improves the
NSC proper motions to $\sim$2.5 mas/yr (precision) and $\sim$0.2 mas/yr
(accuracy). The combination of sources into unique objects is performed using a
DBSCAN algorithm and mean parameters per object (such as mean magnitudes,
proper motion, etc.) are calculated more robustly with outlier rejection.
Finally, eight multi-band photometric variability indices are calculated for
each object and variable objects are flagged (23 million objects). NSC DR2 will
be useful for exploring solar system objects, stellar streams, dwarf satellite
galaxies, QSOs, variable stars, high-proper motion stars, and transients.
Several examples of these science use cases are presented. The NSC DR2 catalog
is publicly available via the NOIRLab's Astro Data Lab science platform.

Astrophysics of Galaxies