  For Gaia DR2 (GDR2), 280 million spectra, collected by the RVS instrument
on-board Gaia, were processed and median radial velocities were derived for 9.8
million sources brighter than Grvs = 12 mag. This paper describes the
validation and properties of the median radial velocities published in GDR2.
Quality tests and filters are applied to select, from the 9.8 million radial
velocities, those with the quality to be published in GDR2. The accuracy of the
selected sample is assessed with respect to ground-based catalogues. Its
precision is estimated using both ground-based catalogues and the distribution
of the Gaia radial velocity uncertainties. GDR2 contains median radial
velocities for 7 224 631 stars, with Teff in the range [3550, 6900] K, which
passed succesfully the quality tests. The published median radial velocities
provide a full sky-coverage and have a completness with respect to the
astrometric data of 77.2\% (for $G \leq 12.5$ mag). The median radial velocity
residuals with respect to the ground-based surveys vary from one catalogue to
another, but do not exceed a few 100s m/s. In addition, the Gaia radial
velocities show a positive trend as a function of magnitude, which starts
around Grvs $\sim 9$ mag and reaches about $+500$ m/s at Grvs $= 11.75$ mag.
The overall precision, estimated from the median of the Gaia radial velocity
uncertainties, is 1.05 km/s. The radial velocity precision is function of many
parameters, in particular the magnitude and effective temperature. For bright
stars, Grvs in [4, 8] mag, the precision is in the range 200-350 m/s, which is
about 3 to 5 times more precise than the pre-launch specification of 1 km/s. At
the faint end, Grvs = 11.75 mag, the precisions for Teff = 5000 K and 6500 K
are respectively 1.4 km/s and 3.7 km/s.
