  We present a catalog of 182 galaxy clusters detected through the
Sunyaev-Zel'dovich effect by the Atacama Cosmology Telescope in a contiguous
987.5 deg$^{2}$ field. The clusters were detected as SZ decrements by applying
a matched filter to 148 GHz maps that combine the original ACT equatorial
survey with data from the first two observing seasons using the ACTPol
receiver. Optical/IR confirmation and redshift measurements come from a
combination of large public surveys and our own follow-up observations. Where
necessary, we measured photometric redshifts for clusters using a pipeline that
achieves accuracy $\Delta z/(1 + z)=0.015$ when tested on SDSS data. Under the
assumption that clusters can be described by the so-called Universal Pressure
Profile and its associated mass-scaling law, the full signal-to-noise > 4
sample spans the mass range $1.6 < M^{\rm UPP}_{\rm 500c}/10^{14}{\rm
M}_{\odot}<9.1$, with median $M^{\rm UPP}_{\rm 500c}=3.1 \times 10^{14}$
M$_{\odot}$. The sample covers the redshift range $0.1 < z < 1.4$ (median $z =
0.49$) and 28 clusters are new discoveries (median $z = 0.80$). We compare our
catalog with other overlapping cluster samples selected using the SZ,
optical,and X-ray wavelengths. We find the ratio of the UPP-based SZ mass to
richness-based weak-lensing mass is $\langle M^{\rm UPP}_{\rm 500c} \rangle /
\langle M^{\rm \lambda WL}_{\rm 500c} \rangle = 0.68 \pm 0.11$. After applying
this calibration, the mass distribution for clusters with $M_{\rm 500c} > 4
\times 10^{14}$ M$_{\odot}$ is consistent with the number of such clusters
found in the South Pole Telescope SZ survey.
