  The third catalog of active galactic nuclei (AGNs) detected by the Fermi-LAT
(3LAC) is presented. It is based on the third Fermi-LAT catalog (3FGL) of
sources detected between 100 MeV and 300 GeV with a Test Statistic (TS) greater
than 25, between 2008 August 4 and 2012 July 31. The 3LAC includes 1591 AGNs
located at high Galactic latitudes (|b|>10{\deg}), a 71% increase over the
second catalog based on 2 years of data. There are 28 duplicate associations,
thus 1563 of the 2192 high-latitude gamma-ray sources of the 3FGL catalog are
AGNs. Most of them (98%) are blazars. About half of the newly detected blazars
are of unknown type, i.e., they lack spectroscopic information of sufficient
quality to determine the strength of their emission lines. Based on their
gamma-ray spectral properties, these sources are evenly split between
flat-spectrum radio quasars (FSRQs) and BL~Lacs. The most abundant detected
BL~Lacs are of the high-synchrotron-peaked (HSP) type. About 50% of the BL~Lacs
have no measured redshifts. A few new rare outliers (HSP-FSRQs and
high-luminosity HSP BL~Lacs) are reported. The general properties of the 3LAC
sample confirm previous findings from earlier catalogs. The fraction of 3LAC
blazars in the total population of blazars listed in BZCAT remains
non-negligible even at the faint ends of the BZCAT-blazar radio, optical and
X-ray flux distributions, which is a clue that even the faintest known blazars
could eventually shine in gamma rays at LAT-detection levels. The energy-flux
distributions of the different blazar populations are in good agreement with
extrapolation from earlier catalogs.
