  Characterizing the bulk atmosphere of a terrestrial planet is important for
determining surface pressure and potential habitability. Molecular nitrogen
(N$_2$) constitutes the largest fraction of Earth$'$s atmosphere and is likely
to be a major constituent of many terrestrial exoplanet atmospheres. Due to its
lack of significant absorption features, N$_2$ is extremely difficult to
remotely detect. However, N$_2$ produces an N$_2$-N$_2$ collisional pair,
(N$_2$)$_2$, which is spectrally active. Here we report the detection of
(N$_2$)$_2$ in Earth$'$s disk-integrated spectrum. By comparing spectra from
NASA$'$s EPOXI mission to synthetic spectra from the NASA Astrobiology
Institute$'$s Virtual Planetary Laboratory three-dimensional spectral Earth
model, we find that (N$_2$)$_2$ absorption produces a ~35$\%$ decrease in flux
at 4.15 $\mu$m. Quantifying N$_2$ could provide a means of determining bulk
atmospheric composition for terrestrial exoplanets and could rule out abiotic
O$_2$ generation, which is possible in rarefied atmospheres. To explore the
potential effects of (N$_2$)$_2$ in exoplanet spectra, we used radiative
transfer models to generate synthetic emission and transit transmission spectra
of self-consistent N$_2$-CO$_2$-H$_2$O atmospheres, and analytic N$_2$-H$_2$
and N$_2$-H$_2$-CO$_2$ atmospheres. We show that (N$_2$)$_2$ absorption in the
wings of the 4.3 $\mu$m CO$_2$ band is strongly dependent on N$_2$ partial
pressures above 0.5 bar and can significantly widen this band in thick N$_2$
atmospheres. The (N$_2$)$_2$ transit transmission signal is up to 10 ppm for an
Earth-size planet with an N$_2$-dominated atmosphere orbiting within the HZ of
an M5V star and could be substantially larger for planets with significant
H$_2$ mixing ratios.
