  The second catalog of active galactic nuclei (AGNs) detected by the Fermi
Large Area Telescope (LAT) in two years of scientific operation is presented.
The Second LAT AGN Catalog (2LAC) includes 1017 gamma-ray sources located at
high Galactic latitudes (|b|>10{\deg}) that are detected with a test statistic
greater than 25 and associated statistically with AGNs. However some of these
are affected by analysis issues and some are associated with multiple AGNs.
Consequently we define a clean sample which includes 886 AGNs, comprising 395
BL Lacertae objects (BL Lacs), 310 flat-spectrum radio quasars (FSRQs), 157
candidate blazars of unknown type (i.e., with broad-band blazar characteristics
but with no optical spectral measurement yet), eight misaligned AGNs, four
narrow-line Seyfert 1 (NLS1s), 10 AGNs of other types and two starburst
galaxies. Where possible, the blazars have been further classified based on
their spectral energy distributions (SEDs) as archival radio, optical, and
X-ray data permit. While almost all FSRQs have a synchrotron-peak frequency
10^14 Hz, about half of the BL Lacs have a synchrotron-peak frequency >10^15
Hz. The 2LAC represents a significant improvement relative to the First LAT AGN
Catalog (1LAC), with 52% more associated sources. The full characterization of
the newly detected sources will require more broad-band data. Various
properties, such as gamma-ray fluxes and photon power law spectral indices,
redshifts, gamma-ray luminosities, variability, and archival radio
luminosities---and their correlations are presented and discussed for the
different blazar classes. The general trends observed in 1LAC are confirmed.
