irregular

Short definition from Wiktionary

Adjective

1. 1
   Nonstandard; not conforming to rules or expectations.
2. 2
   Rough (of a surface).
3. 3
   Without symmetry, regularity, or uniformity.
4. 4
   Not regular; having sides that are not equal or angles that are not equal.
5. 5
   Whose faces are not all regular polygons (or are not equally inclined to each other).
6. 6
   Not following the regular or expected patterns of inflection in a given language.

Noun

1. 1
   A soldier who is not a member of an official military force and who may not use regular army tactics.
2. 2
   One who does not regularly attend a venue.