A soldier who is not a member of an official military force and who may not use 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

(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)

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