  A measurement of electron antineutrino oscillation by the Daya Bay Reactor
Neutrino Experiment is described in detail. Six 2.9-GW$_{\rm
  th}$ nuclear power reactors of the Daya Bay and Ling Ao nuclear power
facilities served as intense sources of $\overline{\nu}_{e}$'s. Comparison of
the $\overline{\nu}_{e}$ rate and energy spectrum measured by antineutrino
detectors far from the nuclear reactors ($\sim$1500-1950 m) relative to
detectors near the reactors ($\sim$350-600 m) allowed a precise measurement of
$\overline{\nu}_{e}$ disappearance. More than 2.5 million $\overline{\nu}_{e}$
inverse beta decay interactions were observed, based on the combination of 217
days of operation of six antineutrino detectors (Dec. 2011--Jul. 2012) with a
subsequent 1013 days using the complete configuration of eight detectors (Oct.
2012--Jul. 2015). The $\overline{\nu}_{e}$ rate observed at the far detectors
relative to the near detectors showed a significant deficit, $R=0.949 \pm
0.002(\mathrm{stat.}) \pm 0.002(\mathrm{syst.})$. The energy dependence of
$\overline{\nu}_{e}$ disappearance showed the distinct variation predicted by
neutrino oscillation. Analysis using an approximation for the three-flavor
oscillation probability yielded the flavor-mixing angle
$\sin^22\theta_{13}=0.0841 \pm 0.0027(\mathrm{stat.}) \pm
0.0019(\mathrm{syst.})$ and the effective neutrino mass-squared difference of
$\left|{\Delta}m^2_{\mathrm{ee}}\right|=(2.50 \pm 0.06(\mathrm{stat.}) \pm
0.06(\mathrm{syst.})) \times 10^{-3}\ {\rm eV}^2$. Analysis using the exact
three-flavor probability found ${\Delta}m^2_{32}=(2.45 \pm 0.06(\mathrm{stat.})
\pm 0.06(\mathrm{syst.})) \times 10^{-3}\ {\rm eV}^2$ assuming the normal
neutrino mass hierarchy and ${\Delta}m^2_{32}=(-2.56 \pm 0.06(\mathrm{stat.})
\pm 0.06(\mathrm{syst.})) \times 10^{-3}\ {\rm eV}^2$ for the inverted
hierarchy.
