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.

Military . (formerly, of troops) not belonging to an organized group of the established forces.

flawed, damaged, or failing to meet a specific standard of manufacture

a sale of irregular shirts.

Botany .

not uniform.

(of a flower) having the members of some or all of its floral circles or whorls differing from one another in size or shape, or extent of union.

deviating or experiencing deviations from a normally regular or cyclic body function, as bowel habits or menstruation.

(of a stock or commodity market) of mixed market activity; showing no clear up or down trend.

a person or thing that is irregular.

Commerce . a product or material that does not meet specifications or standards of the manufacturer, as one having imperfections in its pattern.

Military . a soldier or combatant not of a regular military force, as a guerrilla or partisan.

adjective

conventional , methodical , regular , systematic , common , normal , standard , usual , even , flat , level , smooth

adjective

aberrant , aimless , capricious , casual , changeable , designless , desultory , disconnected , discontinuous , eccentric , erratic , faltering , fitful , fluctuating , fragmentary , haphazard , hit-or-miss * , inconstant , indiscriminate , infrequent , intermittent , jerky , nonuniform , occasional , out of order , patchy , purposeless , recurrent , shaky , shifting , spasmodic , sporadic , uncertain , unconsidered , uneven , unmethodical , unpunctual , unreliable , unsettled , unsteady , unsystematic , up and down , weaving , anomalous , atypical , deviant , different , disorderly , divergent , exceptional , extraordinary , immoderate , improper , inappropriate , inordinate , odd , off-key * , queer , quirky , singular , strange , unconventional , unique , unnatural , unofficial , unorthodox , unsuitable , unusual , amorphous , asymmetrical , bent , broken , cockeyed * , craggy , crooked , devious , disproportionate , elliptic , elliptical , hilly , jagged , lopsided , lumpy , meandering , notched , not uniform , off-balance , off-center , out of proportion , pitted , protuberant , rough , scarred , serrate , serrated , unaligned , unbalanced , unequal , unsymmetrical , variable , wobbly , zigzagged , asymmetric , anomalistic , atypic , preternatural , abnormal , amiss , arrhythmic , baroque , clandestine , cursory , distorted , diverse , episodic , erose , heteroclite , heteromorphic , imperfect , inconformable , inconsistent , nonconforming , peculiar , promiscuous , rambling , random , spotty , straggly , tumultuary , uncontrolled , unlike , unstable , wayward