A civilian is a person who is not a member of a military. Under the Fourth Geneva Convention it is a war crime to deliberately attack a military objective; civilian property may be seized for military use; and collateral damage is an accepted part of war.
In practice, the neat division between combatants and non-combatants implicit in such treaties can be very unclear, particularly in guerilla warfare where the guerillas receive the support of the local population. It is sometimes argued that the division between civilian and military and the abhorrence towards attacks on civilians is a reflection of Western attitudes to war, and that other societies do not make such distinctions but find other aspects of Western-style warfare abhorrent (such as strategic bombing).
The possible cultural perception notwithstanding, 188 states are party to the appplicable Geneva Convention (as at 31 December 1996) including such non-Western countries that have been in conflict since the 12 August 1949 date of the Convention as Afghanistan, Cambodia, China, Congo, Iran, Iraq, Jordan, both Koreas, Kuwait, Laos, Rwanda, Vietnam.  (http://www.icrc.org/Web/eng/siteeng0.nsf/iwpList160/D89499AA0F2E88C0C1256B66005A784B)
See also: Laws of war, Law of land warfare, combatant. illegal combatant.
In legal contexts, a civilian is also the name given to a jurist or jurisdiction of the civil law as in civilian jurisdiction.
In interactions between certain professions, especially such public service ones as police and fire protection, a civilian also describes a person not part of the profession. "Civilian" is also known to be used by models, federal bureaucrats, and historically, by vaudevillians to describe those outside of their respective professions. Term recently gained increased parlance through use by New York Theater Company, "The Civilians"  (http://www.thecivilians.org).
In the book and movie Starship Troopers, the social class is divided between citizens and civilians. Citizens are discribed as people who are willing to make the safety of mankind their own responsibility. Civilians don't have that courage.