The Collaborative International Dictionary of English v.0.48:

Irregularity \Ir*reg`u*lar"i*ty\, n.; pl. Irregularities. [Cf.
   F. irr['e]gularit['e].]
   The state or quality of being irregular; that which is
   irregular.
   [1913 Webster]

WordNet (r) 3.0 (2006):

irregularity
    n 1: behavior that breaches the rule or etiquette or custom or
         morality [syn: abnormality, irregularity]
    2: not characterized by a fixed principle or rate; at irregular
       intervals [syn: irregularity, unregularity] [ant:
       regularity]
    3: an irregular asymmetry in shape; an irregular spatial pattern
       [syn: irregularity, geometrical irregularity]
    4: irregular and infrequent or difficult evacuation of the
       bowels; can be a symptom of intestinal obstruction or
       diverticulitis [syn: constipation, irregularity]