  We present late-time radio/millimeter (as well as optical/UV and X-ray)
detections of the tidal disruption event (TDE) AT2018hyz, spanning $970 - 1300$
d after optical discovery. In conjunction with earlier deeper limits, including
at $\approx 700$ d, our observations reveal rapidly rising emission at
$0.8-240$ GHz, steeper than $F_\nu\propto t^5$ relative to the time of optical
discovery. Such a steep rise cannot be explained in any reasonable scenario of
an outflow launched at the time of disruption (e.g., off-axis jet, sudden
increase in the ambient density), and instead points to a delayed launch. Our
multi-frequency data allow us to directly determine the radius and energy of
the radio-emitting outflow, showing that it was launched $\approx 750$ d after
optical discovery. The outflow velocity is mildly relativistic, with
$\beta\approx 0.25$ and $\approx 0.6$ for a spherical and a $10^\circ$ jet
geometry, respectively, and the minimum kinetic energy is $E_K\approx 5.8\times
10^{49}$ and $\approx 6.3\times 10^{49}$ erg, respectively. This is the first
definitive evidence for the production of a delayed mildly-relativistic outflow
in a TDE; a comparison to the recently-published radio light curve of
ASASSN-15oi suggests that the final re-brightening observed in that event (at a
single frequency and time) may be due to a similar outflow with a comparable
velocity and energy. Finally, we note that the energy and velocity of the
delayed outflow in AT2018hyz are intermediate between those of past
non-relativistic TDEs (e.g., ASASSN-14li, AT2019dsg) and the relativistic TDE
Sw\,J1644+57. We suggest that such delayed outflows may be common in TDEs.
