  We report the first plausible optical electromagnetic (EM) counterpart to a
(candidate) binary black hole (BBH) merger. Detected by the Zwicky Transient
Facility (ZTF), the EM flare is consistent with expectations for a kicked BBH
merger in the accretion disk of an active galactic nucleus (AGN), and is
unlikely ($<O(0.01\%$)) due to intrinsic variability of this source. The lack
of color evolution implies that it is not a supernovae and instead is strongly
suggestive of a constant temperature shock. Other false-positive events, such
as microlensing or a tidal disruption event, are ruled out or constrained to be
$<O(0.1\%$). If the flare is associated with S190521g, we find plausible values
of: total mass $ M_{\rm BBH} \sim 100 M_{\odot}$, kick velocity $v_k \sim 200\,
{\rm km}\, {\rm s}^{-1}$ at $\theta \sim 60^{\circ}$ in a disk with aspect
ratio $H/a \sim 0.01$ (i.e., disk height $H$ at radius $a$) and gas density
$\rho \sim 10^{-10}\, {\rm g}\, {\rm cm}^{-3}$. The merger could have occurred
at a disk migration trap ($a \sim 700\, r_{g}$; $r_g \equiv G M_{\rm SMBH} /
c^2$, where $M_{\rm SMBH}$ is the mass of the AGN supermassive black hole). The
combination of parameters implies a significant spin for at least one of the
black holes in S190521g. The timing of our spectroscopy prevents useful
constraints on broad-line asymmetry due to an off-center flare. We predict a
repeat flare in this source due to a re-encountering with the disk in $\sim
1.6\, {\rm yr}\, (M_{\rm SMBH}/10^{8}M_{\odot})\, (a/10^{3}r_{g})^{3/2}$.
