adjective

1. not conforming to established rule, method, usage, standard, etc.; out of the ordinary; anomalous
2. not conforming to legal or moral requirements; lawless; disorderly
3. not straight or even; not symmetrical; not uniform in shape, design, or proportion
4. uneven in occurrence or succession; variable or erratic
5. ☆ having minor flaws or imperfections: said of merchandise
6. experiencing constipation
7. Bot. not uniform in shape, size, etc. as the petals of a flower
8. Gram. not inflected in the usual way “go” is an irregular verb
9. Mil. not belonging to the regularly established army

Etymology: ME < OFr irregulier < ML irregularis: see in- & regular

noun

1. a person or thing that is irregular
2. a soldier who belongs to an irregular military force
3. ☆ irregular merchandise