irregular

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

One who does not regularly attend a venue

Nonstandard; not conforming to rules or expectations

(of a surface) rough

Without symmetry, regularity, or uniformity

(of a polygon) not regular; having sides that are not equal or angles that are not equal

(of a polyhedron) whose faces are not all regular polygons (or are not equally inclined to each other)

(grammar, of a word) not following the regular or expected patterns of inflection in a given language

“"Calves", "cacti", and "children" are irregular plurals.”