Perturbation theory
In mathematics the use of the term "theory" is different, necessarily so, since mathematics contains no explanations of natural phenomena, "per se", even though it may help provide insight into natural systems or be inspired by them. In the general sense, a mathematical "theory" is a branch of or topic in mathematics, such as Set theory, Number theory, Group theory, Probability theory, Game theory, Control theory, Perturbation theory, etc., such as might be appropriate for a single textbook.