Source : Webster's Revised Unabridged Dictionary (1913)

Irregular \Ir*reg"u*lar\, n.
   One who is not regular; especially, a soldier not in regular
   service.

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}.

         Mazes intricate, Eccentric, intervolved, yet regular
         Then most when most irregular they seem. --Milton.

         Leading the men of Herefordshire to fight Against the
         irregular and wild Glendower.            --Shak.

         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.

Source : WordNet®

irregular
     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}]

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}]