  Cosmological probes based on galaxy clusters rely on cluster number counts
and large-scale structure information. X-ray cluster surveys are well suited
for this purpose, since they are far less affected than optical surveys by
projection effects, and cluster properties can be predicted with good accuracy.
The XMM Cluster Archive Super Survey, X-CLASS, is a serendipitous search of
X-ray-detected galaxy clusters in 4176 XMM-Newton archival observations until
August 2015. All observations are clipped to exposure times of 10 and 20 ks to
obtain uniformity and they span ~269 deg$^2$ across the high-Galactic latitude
sky ($|b|> 20^o$). The main goal of the survey is the compilation of a
well-selected cluster sample suitable for cosmological analyses. We describe
the detection algorithm, the visual inspection, the verification process and
the redshift validation of the cluster sample, as well as the cluster selection
function computed by simulations. We also present the various metadata that are
released with the catalogue, along with the redshifts of 124 clusters obtained
with a dedicated multi-object spectroscopic follow-up programme. With this
publication we release the new X-CLASS catalogue of 1646 well-selected
X-ray-detected clusters over a wide sky area, along with their selection
function. The sample spans a wide redshift range, from the local Universe up to
z~1.5, with 982 spectroscopically confirmed clusters, and over 70 clusters
above z=0.8. Because of its homogeneous selection and thorough verification,
the cluster sample can be used for cosmological analyses, but also as a
test-bed for the upcoming eROSITA observations and other current and future
large-area cluster surveys. It is the first time that such a catalogue is made
available to the community via an interactive database which gives access to a
wealth of supplementary information, images, and data.
