  There are many logically and computationally distinct characterizations of
the surface gravity of a horizon, just as there are many logically rather
distinct notions of horizon. Fortunately, in standard general relativity, for
stationary horizons, most of these characterizations are degenerate. However,
in modified gravity, or in analogue spacetimes, horizons may be non-Killing or
even non-null, and hence these degeneracies can be lifted. We present a brief
overview of the key issues, specifically focusing on horizons in analogue
spacetimes and universal horizons in modified gravity.
