  For most of their lifetime, super-massive black holes (SMBHs) commonly found
in galactic nuclei obtain mass from the ambient at a rate well below the
Eddington limit, which is mediated by a radiatively inefficient, hot accretion
flow. Both theory and numerical simulations predict that a strong wind must
exist in such hot accretion flows. The wind is of special interest not only
because it is an indispensable ingredient of accretion, but perhaps more
importantly, it is believed to play a crucial role in the evolution of the host
galaxy via the so-called kinetic mode AGN feedback. Observational evidence for
this wind, however, remains scarce and indirect. Here we report the detection
of a hot outflow from the low-luminosity active galactic nucleus in M81, based
on {\it Chandra} high-resolution X-ray spectroscopy. The outflow is evidenced
by a pair of Fe XXVI Ly$\alpha$ lines redshifted and blueshifted at a bulk
line-of-sight velocity of $\pm2.8\times10^3 \rm~km~s^{-1}$ and a high Fe XXVI
Ly$\alpha$-to-Fe XXV K$\alpha$ line ratio implying a plasma temperature of
$1.3\times10^8$ Kelvin. This high-velocity, hot plasma cannot be produced by
stellar activity or the accretion inflow onto the SMBH. Our
magnetohydrodynamical simulations show, instead, it is naturally explained by a
wind from the hot accretion flow, propagating out to $\gtrsim10^6$ times the
gravitational radius of the SMBH. The kinetic energy and momentum of this wind
can significantly affect the evolution of the circumnuclear environment and
beyond.
