In the mathematical discipline of ergodic theory, a Sinai–Ruelle–Bowen (SRB) measure is an invariant measure that behaves similarly to, but is not an ergodic measure. In order to be ergodic, the time average would need to be equal the space average for almost all initial states x ∈ X {\displaystyle x\in X} , with X {\displaystyle X} being the phase space. For an SRB measure μ {\displaystyle \mu } , it suffices that the ergodicity condition be valid for initial states in a set B ( μ ) {\displaystyle B(\mu )} of positive Lebesgue measure.
The initial ideas pertaining to SRB measures were introduced by Yakov Sinai, David Ruelle and Rufus Bowen in the less general area of Anosov diffeomorphisms and axiom A attractors.