  The gravitational-wave signal GW190521 is consistent with a binary black hole
merger source at redshift 0.8 with unusually high component masses,
$85^{+21}_{-14}\,M_{\odot}$ and $66^{+17}_{-18}\,M_{\odot}$, compared to
previously reported events, and shows mild evidence for spin-induced orbital
precession. The primary falls in the mass gap predicted by (pulsational)
pair-instability supernova theory, in the approximate range $65 -
120\,M_{\odot}$. The probability that at least one of the black holes in
GW190521 is in that range is 99.0%. The final mass of the merger
$(142^{+28}_{-16}\,M_{\odot})$ classifies it as an intermediate-mass black
hole. Under the assumption of a quasi-circular binary black hole coalescence,
we detail the physical properties of GW190521's source binary and its
post-merger remnant, including component masses and spin vectors. Three
different waveform models, as well as direct comparison to numerical solutions
of general relativity, yield consistent estimates of these properties. Tests of
strong-field general relativity targeting the merger-ringdown stages of
coalescence indicate consistency of the observed signal with theoretical
predictions. We estimate the merger rate of similar systems to be
$0.13^{+0.30}_{-0.11}\,{\rm Gpc}^{-3}\,\rm{yr}^{-1}$. We discuss the
astrophysical implications of GW190521 for stellar collapse, and for the
possible formation of black holes in the pair-instability mass gap through
various channels: via (multiple) stellar coalescence, or via hierarchical
merger of lower-mass black holes in star clusters or in active galactic nuclei.
We find it to be unlikely that GW190521 is a strongly lensed signal of a
lower-mass black hole binary merger. We also discuss more exotic possible
sources for GW190521, including a highly eccentric black hole binary, or a
primordial black hole binary.
