In general use, an atrocity or massacre is the moralist's term for a politically or ethnically motivated mass-killing of civilians. In international law, more precise terms are war crime and crime against humanity.
An atrocity can be a single specific event, or a series of events, or can refer to genocide. A defining characteristic of an atrocity is its brutal or systematic nature. It is an act of killing that is in violation of all moral principles, and can only be justified by social systems that are deviantly altered from long established morality. Killing and war has been happenning since before the historical record. Often hostilities exceed the legitimate mandate of killing enemy combatants to include attacks upon unarmed or otherwise non-combative peoples. Thus every culture has in its history acts of killing which are atrocities.
In the last century, over 160 million civilians were killed in violent conflicts, compared with roughly only 40 million soldiers.
Informally, people may refer to individual or a limited number of deaths of a cruel nature as an atrocity, as people may do so to characterise such an act as immoral, rather than to say the death was comparable to larger mass killings.
The word 'atrocity' in use, is often political. One state may refer to the acts of killing of another as atrocities or murder while its own killing is not so regarded, and justified in context.
In the context of a war, civilians are always killed. However, countries have committed to certain laws of war. The degree to which a military is designed to consider the lives of civilians is often given a relative sociopolitical value. Between combatants, the more considerate of the two being the more 'righteous', even though both sides may commit grave acts of mass killing, such acts are often not referred to as 'atrocities.''Atrocity' in political use