In the context of quantum field theory and statistical physics, an "infrared fixed point" refers to a particular type of fixed point in the renormalization group flow where the behavior of the system at long wavelengths (or low energies) becomes scale-invariant. This means that, as one examines the system at larger and larger scales or lower and lower energies, the physical properties of the system do not change—they remain self-similar.
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An infrared fixed point attracts renormalization-group flows toward low energies or long distances. If it is interacting, it defines a nontrivial scale-invariant quantum or statistical field theory.