The traditionally accepted political definition of peace and the very word itself comes from the influence of the ancient Romans who defined peace, pax, as absentia belli, the absence of war.

In the modern nation-state world, peace has often been understood as the absence of international war between two or more state-organized armies. However, constraining the concept of peace to the absence of international war ignores the relevance of civil war, internal genocide, terrorism, and other violence. Since World War II, wars between states have decreased while violent internal conflicts increased. Since 1992, however, internal conflicts have decreased though there are still numerous and problematic intractable conflicts. Present day Sudan, as an example, is not engaged in a war with another sovereign state. However, few would deny that what amounts to genocide is taking place in its Darfur region.

Peace therefore, in the sense of the absence of hostilities, is best understood as the absence of violence between groups, whether they are part of a state apparatus or not. Johan Galtung, a very influential peace researcher, calls peace understood simply as the absence of violence "negative peace." It is labeled as such because although direct violence is not taking place, the points of conflict, which can lead to violence, are not necessarily resolved.