  We present a spectroscopic and photometric analysis of a sample of 416,288
galaxies from the Sloan Digital Sky Survey (SDSS) matched to mid-infrared
(mid-IR) data from the Wide-Field Infrared Survey Explorer (WISE). By using a
new spectroscopic fitting package, GELATO (Galaxy/AGN Emission Line Analysis
TOol), we are able to retrieve emission line fluxes and uncertainties for SDSS
spectra and robustly determine the presence of broad lines and outflowing
components, enabling us to investigate WISE color space as a function of
optical spectroscopic properties. In addition, we pursue SED template fitting
to assess the relative AGN contribution and nuclear obscuration to compare to
existing mid-IR selection criteria with WISE. We present a selection criterion
in mid-IR color space to select Active Galactic Nuclei (AGNs) with a $\sim$80%
accuracy and a completeness of $\sim$16%. This is the first mid-IR color
selection defined by solely using the distribution of Type I and Type II
optical spectroscopic AGNs in WISE mid-IR color space. Our selection is an
improvement of $\sim$50% in the completeness of targeting spectroscopic AGNs
with WISE down to an SDSS $r<17.77$ mag. In addition, our new criterion targets
a less luminous population of AGNs, with on average lower [O III] luminosities
by $\sim$30% ($>0.1$ dex) compared to typical WISE color-color selections. With
upcoming large photometric surveys without corresponding spectroscopy, our
method presents a way to select larger populations of AGNs at lower AGN
luminosities and higher nuclear obscuration levels than traditional mid-IR
color selections.
