Axiomatic quantum field theory
Axiomatic quantum field theory is a mathematical discipline which aims to describe quantum field theory in terms of rigorous axioms. It is strongly associated with functional analysis and operator algebras, but has also been studied in recent years from a more geometric and functorial perspective.
There are two main challenges in this discipline. First, one must propose a set of axioms which describe the general properties of any mathematical object that deserves to be called a "quantum field theory". Then, one give rigorous mathematical constructions of examples satisfying these axioms.
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Analytic approaches
Wightman axioms
The first set of axioms for quantum field theories, known as the Wightman axioms. were proposed by Arthur Wightman in the early 1950s. These axioms attempt to describe QFTs on flat Minkowski spacetime by regarding quantum fields as operator-valued distributions acting on a Hilbert space. In practice, one often uses the Wightman reconstruction theorem, which guarantees that the operator-valued distributions and the Hilbert space can be recovered from the collection of correlation functions.
Osterwalder-Schrader axioms
The correlation functions of a QFT satisfying the Wightman axioms often can be analytically continued from Lorentz signature to Euclidean signature. (Crudely, one replaces the time variable 
with imaginary time 
; the factors of 
change the sign of the time-time components of the metric tensor.) The resulting functions are called Schwinger functions. The Schwinger functions are a list of conditions—analycity, permutation symmetry, Euclidean covariance, and reflection positivity—which a set of functions defined on various powers of Euclidean spacetime must satisfy in order to be the analytic continuation of the set of correlation functions of a QFT satisfying the Wightman axioms.
Haag-Kastler axioms
The Local quantum physics axiomatize QFT in terms of nets of algebras.