  The 2012 brightening of SN 2009ip was dominated by emission from the
interaction of ejecta with the surrounding circumstellar material (CSM)
produced by episodic mass loss from the progenitor, complicating the diagnosis
of whether the underlying explosion was a true supernova or a nonterminal
eruption of a massive star. In this paper, we contribute a time series of
optical photometric and spectroscopic observations for SN 2009ip from 1 to 3
years after the 2012 outburst, collected at the Las Cumbres Observatory and the
Keck Observatory. We find that the brightness of SN 2009ip continues to decline
with no deviations from a linear slope of $0.0030\pm0.0005$ $\rm mag\ day^{-1}$
in the $r^{\prime}$ band, and demonstrate that this is similar to both
observations and models of CSM-ejecta interaction. We show that the late-time
spectra continue to be dominated by the signature features of CSM interaction,
and that the large ratio of $L_{\rm H\alpha}/L_{\rm H\beta}\approx40$ implies
that the material remains optically thick to Balmer photons ("Case C"
recombination). We combine our late-time photometry and spectra with early-time
data for SN 2009ip and provide a comprehensive discussion that incorporates
recently published models and observations for transient phenomena dominated by
CSM-ejecta interaction, and conclude that the presence of broad H$\alpha$ at
early times remains among the best evidence that a terminal supernova has
occurred. Finally, we compare our late-time spectra to those of Type IIn SNe
and SN impostors at late phases and find that although SN 2009ip has some
similarities with both types, it has more differences with late-time impostor
spectra.
