Noun
perturbation theory (uncountable)
English Wikipedia has an article on:perturbation theoryWikipedia
A mathematical technique for finding an approximate solution to a problem that cannot be solved exactly, by starting from the exact solution of a related problem; used in modelling physical interactions between particles, etc.
Perturbation theory gives well-defined predictions only in renormalizable quantum field theories, and luckily QED, the first fully developed QFT, belonged to this class of renormalizable theories. Source: Internet