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 non-combatant civilian or wantonly and unnecessarily destroy or take the property of a civilian.
In practice, the neat division between combatants and non-combatants implicit in such treaties can get very blurry, 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).
A civilian is also the name given to a jurist or jurisdiction of the civil law as in civilian jurisdiction.