From WordNet (r) 2.0 (August 2003) [wn]:
irregular
adj 1: contrary to rule or accepted order or general practice;
"irregular hiring practices" [ant: regular]
2: (of solids) not having clear dimensions that can be
measured; volume must be determined with the principle of
liquid displacement [ant: regular]
3: not occurring at expected times [syn: unpredictable]
4: used of the military; not belonging to or engaged in by
regular army forces; "irregular troops"; "irregular
warfare" [ant: regular]
5: deviating from what is usual or common or to be expected;
often somewhat odd or strange; "these days large families
are atypical"; "highly irregular behavior" [syn:
atypical]
6: lacking continuity or regularity; "an irregular worker";
"employed on a temporary basis" [syn: temporary]
7: of a surface; not level or flat; "walking was difficult on
the irregular cobblestoned surface"
8: used of independent armed resistance forces; "guerrilla
warfare"; "partisan forces" [syn: {guerrilla(a)},
{guerilla(a)}, underground]
9: independent in behavior or thought; "she led a somewhat
irregular private life"; "maverick politicians" [syn:
maverick, unorthodox]
n 1: a member of an irregular armed force that fights a stronger
force by sabotage and harassment [syn: guerrilla,
guerilla, insurgent]
2: merchandise that has imperfections; usually sold at a
reduced price without the brand name [syn: second]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Irregular \Ir*reg"u*lar\, a. [Pref. ir- not + regular: cf. F.
irr['e]gulier.]
Not regular; not conforming to a law, method, or usage
recognized as the general rule; not according to common form;
not conformable to nature, to the rules of moral rectitude,
or to established principles; not normal; unnatural;
immethodical; unsymmetrical; erratic; no straight; not
uniform; as, an irregular line; an irregular figure; an
irregular verse; an irregular physician; an irregular
proceeding; irregular motion; irregular conduct, etc. Cf.
Regular.
[1913 Webster]
Mazes intricate,
Eccentric, intervolved, yet regular
Then most when most irregular they seem. --Milton.
[1913 Webster]
Leading the men of Herefordshire to fight
Against the irregular and wild Glendower. --Shak.
[1913 Webster]
A flowery meadow through which a clear stream murmured
in many irregular meanders. --Jones.
Syn: Immethodical; unsystematic; abnormal; unnatural;
anomalous; erratic; devious; crooked; eccentric;
unsettled; uneven; variable; changeable; mutable;
desultory; disorderly; wild; immoderate; intemperate;
inordinate; vicious.
[1913 Webster]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Irregular \Ir*reg"u*lar\, n.
One who is not regular; especially, a soldier not in regular
service.
[1913 Webster]