Osterwalder-Schrader axioms - Wightman Axioms
  The mathematical axiom systems for quantum field theory grew out of Hilbert's sixth problem, that of stating the problems of quantum theory in precise mathematical terms. There have been several competing mathematical systems of axioms, and here we shall deal with those of A.S. Wightman and of K. Osterwalder and R. Schrader, stated in historical order. They are centered around group symmetry, relative to unitary representations of Lie groups in Hilbert space. We also mention how the Osterwalder--Schrader axioms have influenced the theory of unitary representations of groups. 
Palle E.T. Jorgensen and Gestur \'Olafsson