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A propagator is a two-point kernel describing propagation and, in a free theory, a Green function for the quadratic kinetic operator. Its boundary prescription determines its type: the time-ordered Feynman propagator uses an prescription, while retarded and advanced propagators impose causal boundary conditions.
A **propagator** is a concept used in various fields, particularly in physics and mathematics, with specific meanings depending on the context: 1. **Quantum Field Theory (QFT)**: In the context of quantum field theory, a propagator is a mathematical function that describes the behavior of particles as they propagate from one point to another in spacetime. It essentially provides a mechanism to account for the interactions and effects of fields and particles.