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 $overline{ u}_{e}$s. Comparison of the $overline{ u}_{e}$ 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 $overline{ u}_{e}$ disappearance. More than 2.5 million $overline{ u}_{e}$ 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 $overline{ u}_{e}$ 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 $overline{ u}_{e}$ disappearance showed the distinct variation predicted by neutrino oscillation. Analysis using an approximation for the three-flavor oscillation probability yielded the flavor-mixing angle $sin^22theta_{13}=0.0841 pm 0.0027(mathrm{stat.}) pm 0.0019(mathrm{syst.})$ 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.
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