Reflection positivity is a positivity condition on the correlation functions of a Euclidean quantum field theory. Let
denote reflection of the Euclidean time coordinate. In a functional-integral formulation,
the condition requires
for suitable functionals supported at positive Euclidean times, with the corresponding
sesquilinear version for linear combinations of functionals.
Reflection positivity is the positivity axiom in the Osterwalder-Schrader axioms. It permits the reconstructed state space to carry a positive semidefinite inner product, which is essential for obtaining a quantum theory on Minkowski space from its Schwinger functions.