  We discuss the different definitions of the mass of a halo in common use and
how one may convert between them. Using N-body simulations we show that mass
estimates based on spherical averages are much more tightly correlated with
each other than with masses based on the number of particles in a halo. The
mass functions pertaining to some different mass definitions are estimated and
compared to the `universal form' of Jenkins et al. (2000). Using a different
simulation pipeline and a different cosmological model we show that the mass
function is well fit by the Jenkins et al. (2000) fitting function,
strengthening the claim to universality made by those authors. We show that
care must be taken to match the definitions of mass when using large N-body
simulations to bootstrap scaling relations from smaller hydrodynamical runs to
avoid observationally significant bias in the predictions for abundances of
objects.
