Irregular

From Old French irreguler, from Medieval Latin irregularis, from in- + regularis.

Full definition of irregular

1944, Miles Burton, The Three Corpse Trick Chapter 5, The hovel stood in the centre of what had once been a vegetable garden, but was now a patch of rank weeds. Surrounding this, almost like a zareba, was an irregular ring of gorse and brambles, an unclaimed vestige of the original common.
2013-01-01, Paul Bartel, Ashli Moore, Avian Migration: The Ultimate Red-Eye Flight, Many of these classic methods are still used, with some modern improvements. For example, with the aid of special microphones and automated sound detection software, ornithologists recently reported … that pine siskins (Spinus pinus) undergo an irregular, nomadic type of nocturnal migration.
2013-07-19, Ian Sample, Irregular bedtimes may affect children's brains, Irregular bedtimes may disrupt healthy brain development in young children, according to a study of intelligence and sleeping habits. ¶ Going to bed at a different time each night affected girls more than boys, but both fared worse on mental tasks than children who had a set bedtime, researchers found.

(geometry, of a polygon) Not regular; having sides that are not equal or angles that are not equal.
(geometry, of a polyhedron) Whose faces are not all regular polygons (or are not equally inclined to each other).

(plural irregulars)

A soldier who is not a member of an official military force and, often, does not follow regular army tactics.