  The problems of coarse-graining and averaging of inhomogeneous cosmologies,
and their backreaction on average cosmic evolution, are reviewed from a
physical viewpoint. A particular focus is placed on comparing different notions
of average spatial homogeneity, and on the interpretation of observational
results. Among the physical questions we consider are: the nature of an average
Copernican principle, the role of Mach's principle, the issue of quasilocal
gravitational energy and the different roles of spacetime, spatial and null
cone averages. The observational interpretation of the timescape scenario is
compared to other approaches to cosmological averaging, and outstanding
questions are discussed.
