irregular

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Contrary to rule, accepted order, or general practice.
• adjective Not conforming to legality, moral law, or social convention.
• adjective Not straight, uniform, or symmetrical.
• adjective Of uneven rate, occurrence, or duration.
• adjective Deviating from a type; atypical.
• adjective Botany Having differing floral parts, as of a zygomorphic or asymmetric flower.
• adjective Falling below the manufacturer's standard or usual specifications; imperfect.
• adjective Grammar Departing from the usual pattern of inflection, derivation, or word formation, as the present forms of the verb be or the plural noun children.
• adjective Not belonging to a permanent, organized military force.
• noun One, such as an item of merchandise, that is irregular.
• noun A soldier, such as a guerrilla, who is not a member of a regular military force.

from The Century Dictionary.

• Not regular; lacking regularity or method in some respect; not conformable to rule, order, symmetry, uniformity, or a fixed principle; deviating from the normal or usual course or state; devious; unmethodical; uneven: as, an irregular figure, outline, or surface; irregular verbs; irregular troops.
• Not regular in action or method; not conformed or conforming to regular rules or principles; hence, disorderly; lawless; improper: as, he is given to irregular courses.
• Specifically In human anatomy, being of no determinate shape, as a vertebra: said only of bones.
• In zoology: Not having a definite form; bilaterally or radially unsymmetrical; not having the form usual in a group; differing in an unusual manner from neighboring parts: as, an irregular third joint of an insect's antenna.
• Not arranged in a definite manner, or varying in position or direction: as, irregular marks (that is, marks varying in size or distance from one another); irregular punctures or striæ.
• In echinoderms, not exhibiting radial symmetry; exocyclic or petalostichous; spatangoid or clypeastroid: specifically said of the heart-urchins and other sea-urchins of the division Irregularia. See cut under petalostichous.
• In botany, not having all the members of the same part alike: said of flowers.
• noun One who is not subject or does not conform to established regulations; especially, a soldier who is not in regular service, or a person practising medicine without belonging to the regular profession.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who is not regular; especially, a soldier not in regular service.
• adjective Not regular; not conforming to a law, method, or usage recognized as the general rule; not according to common form; not conformable to nature, to the rules of moral rectitude, or to established principles; not normal; unnatural; immethodical; unsymmetrical; erratic; no straight; not uniform

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Nonstandard; not conforming to rules or expectations.
• adjective Of a surface, rough.
• noun A soldier who is not a member of an official military force and, often, does not follow regular army tactics.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective independent in behavior or thought
• adjective (used of the military) not belonging to or engaged in by regular army forces
• adjective lacking continuity or regularity
• adjective falling below the manufacturer's standard
• adjective (of solids) not having clear dimensions that can be measured; volume must be determined with the principle of liquid displacement
• adjective (of a surface or shape); not level or flat or symmetrical
• noun a member of an irregular armed force that fights a stronger force by sabotage and harassment
• adjective deviating from normal expectations; somewhat odd, strange, or abnormal
• adjective not occurring at expected times
• noun merchandise that has imperfections; usually sold at a reduced price without the brand name
• adjective contrary to rule or accepted order or general practice