  We present the second catalog of high-energy gamma-ray sources detected by
the Large Area Telescope (LAT), the primary science instrument on the Fermi
Gamma-ray Space Telescope (Fermi), derived from data taken during the first 24
months of the science phase of the mission, which began on 2008 August 4.
Source detection is based on the average flux over the 24-month period. The
Second Fermi-LAT catalog (2FGL) includes source location regions, defined in
terms of elliptical fits to the 95% confidence regions and spectral fits in
terms of power-law, exponentially cutoff power-law, or log-normal forms. Also
included are flux measurements in 5 energy bands and light curves on monthly
intervals for each source. Twelve sources in the catalog are modeled as
spatially extended. We provide a detailed comparison of the results from this
catalog with those from the first Fermi-LAT catalog (1FGL). Although the
diffuse Galactic and isotropic models used in the 2FGL analysis are improved
compared to the 1FGL catalog, we attach caution flags to 162 of the sources to
indicate possible confusion with residual imperfections in the diffuse model.
The 2FGL catalog contains 1873 sources detected and characterized in the 100
MeV to 100 GeV range of which we consider 127 as being firmly identified and
1170 as being reliably associated with counterparts of known or likely
gamma-ray-producing source classes.
