irregular vs improper

irregular is a noun but improper is not a noun.

Word	Noun	Adjective	Verb	Adverb
irregular	Yes	Yes	No	No
improper	No	Yes	No	No

irregular (noun)	improper (noun)
merchandise that has imperfections; usually sold at a reduced price without the brand name
a member of an irregular armed force that fights a stronger force by sabotage and harassment

irregular (adjective)	improper (adjective)
contrary to rule or accepted order or general practice	not suitable or right or appropriate
(of solids) not having clear dimensions that can be measured; volume must be determined with the principle of liquid displacement	not appropriate for a purpose or occasion
(used of the military) not belonging to or engaged in by regular army forces	not conforming to legality, moral law, or social convention
deviating from normal expectations; somewhat odd, strange, or abnormal
failing to meet a standard of manufacture due to an imperfection
lacking continuity or regularity
not occurring at a regular rate or fixed intervals
independent in behavior or thought
(of a surface or shape); not level or flat or symmetrical