  We adapt the notion of a (relatively) definable subset of Aut(M) when M is a
saturated model to the case Aut(M/A) when M is atomic and strongly
omega-homogeneous over A. We discuss the existence and uniqueness of invariant
measures on the Boolean algebra of definable subsets of Aut(M/A). For example
when Th(M) is stable we have existence and uniqueness. We also discuss the
compatibility of our definability notions with definable Galois cohomology and
differential Galois theory.
