  We present optical light curves, redshifts, and classifications for 365
spectroscopically confirmed Type Ia supernovae (SNe Ia) discovered by the
Pan-STARRS1 (PS1) Medium Deep Survey. We detail improvements to the PS1 SN
photometry, astrometry and calibration that reduce the systematic uncertainties
in the PS1 SN Ia distances. We combine the subset of 279 PS1 SN Ia ($0.03 < z <
0.68$) with useful distance estimates of SN Ia from SDSS, SNLS, various low-z
and HST samples to form the largest combined sample of SN Ia consisting of a
total of 1048 SN Ia ranging from $0.01 < z < 2.3$, which we call the `Pantheon
Sample'. When combining Planck 2015 CMB measurements with the Pantheon SN
sample, we find $\Omega_m=0.307\pm0.012$ and $w = -1.026\pm0.041$ for the wCDM
model. When the SN and CMB constraints are combined with constraints from BAO
and local H0 measurements, the analysis yields the most precise measurement of
dark energy to date: $w0 = -1.007\pm 0.089$ and $wa = -0.222 \pm0.407$ for the
w0waCDM model. Tension with a cosmological constant previously seen in an
analysis of PS1 and low-z SNe has diminished after an increase of $2\times$ in
the statistics of the PS1 sample, improved calibration and photometry, and
stricter light-curve quality cuts. We find the systematic uncertainties in our
measurements of dark energy are almost as large as the statistical
uncertainties, primarily due to limitations of modeling the low-redshift
sample. This must be addressed for future progress in using SN Ia to measure
dark energy.
