without symmetry, even shape, formal arrangement, etc.: an irregular pattern.
not characterized by any fixed principle, method, continuity, or rate: irregular intervals.
not conforming to established rules, customs, etiquette, morality, etc.: highly irregular behavior.
not according to rule, or to the accepted principle, method, course, order, etc.
Grammar. not conforming to the prevalent pattern or patterns of formation, inflection, construction, etc., of a language; having a rule descriptive of a very small number of items: The English verbs “keep” and “see” are irregular in their inflections.