irregular:
(plural: ‘irregulars‘; pronunciation: the letter rhymes with the letter ‘i’ in “ink”; the opposite word in meaning: ‘regular’ — for the right pronunciation, please click here)
a soldier in a non-regular army which is not part of the official army of a country;
a guerrilla;
(usually in plural — ‘irregulars‘ and less commonly used) a person who buys things from a shop only now and then; not like a “regular” who buys things every week, every month, etc.),

army irregulars planning an attack
Being angry with the life he had been having, Dan joined the army as an irregular and was killed in a battle in the first month.
There are some disgruntled military officers or politicians who turn against the government using the irregular army, a band of irregulars chosen from the scum of the society, and some of them succeed with very little to gain and some of them fail with even less to lose.
Though it is the common belief that army irregulars and guerrillas are almost the same, some military analysts opinion that these two groups fall under different categories, especially in applying fighting tactics.
Most local shops in our town depend on the regulars, townspeople who do shopping in those shops regularly, but there are some shops that sell novelties and rare things that depend mostly on the irregulars, those tourists or travellers that visit the shops only now and then.

an irregular shape
