casual

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

• adjective Being without ceremony or formality; relaxed or informal.
• adjective Unpremeditated or offhand.
• adjective Suited for everyday wear or informal use.
• adjective Not serious or thorough; superficial.
• adjective Showing little interest or concern; nonchalant.
• adjective Occurring or being such by chance: synonym: chance.
• adjective Occurring at irregular or infrequent intervals; occasional.
• adjective Employed on an irregular basis.
• adjective Socialized with only occasionally; not close or intimate.
• adjective Being or occurring between sexual partners who are not in an established relationship.
• noun One that serves or appears at irregular intervals, especially a temporary worker.
• noun Casualwear.
• noun A soldier temporarily attached to a unit while awaiting permanent assignment.

from The Century Dictionary.

• Happening or coming to pass without (apparent) cause, without design on the part of the agent, in an unaccountable manner, or as a mere coincidence or accident; coming by chance; accidental; fortuitous; indeterminate: as, a casual encounter.
• Occasional; coming at uncertain times, or without regularity, in distinction from stated or regular; incidental: as, casual expenses.
• noun A person who receives relief and shelter for one night at the most in a workhouse or police-station, or who receives treatment in a hospital for an accidental injury.
• noun A laborer or an artisan employed only irregularly.

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

• adjective Happening or coming to pass without design, and without being foreseen or expected; accidental; fortuitous; coming by chance.
• adjective Coming without regularity; occasional; incidental.
• noun One who receives relief for a night in a parish to which he does not belong; a vagrant.

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

• adjective Happening by chance.
• adjective Coming without regularity; occasional or incidental.
• adjective Employed irregularly
• adjective Careless.
• adjective Happening or coming to pass without design.
• adjective Informal, relaxed.
• adjective Designed for informal use.
• noun UK, New Zealand A worker who is only working for a company occasionally, not as its permanent employee.
• noun A soldier temporarily at a place of duty, usually en route to another place of duty.
• noun UK A member of a group of football hooligans who wear expensive designer clothing to avoid police attention; see Casual (subculture).
• noun One who receives relief for a night in a parish to which he does not belong; a vagrant.

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

• adjective occurring or appearing or singled out by chance
• adjective natural and unstudied
• adjective hasty and without attention to detail; not thorough
• adjective not showing effort or strain
• adjective characterized by a feeling of irresponsibility
• adjective occurring from time to time
• adjective without or seeming to be without plan or method; offhand
• adjective appropriate for ordinary or routine occasions
• adjective marked by blithe unconcern