Definition of irregular:

part of speech: noun

A soldier not in regular service; one not following any fixed rule; in bot, a flower in which the parts of any of the verticils differ in size.

part of speech: adjective

Not according to usual forms or rules; not according to established principles or customs; not in conformity to law; wanting symmetry; not regular, as regards mode of life; vicious.

part of speech: noun

Irregularity.