The original mandate of traditional UN Peacekeeping Operations, which still constitute the core of Peacekeeping today, is defined as follows by Paul Diehl:

"The imposition of neutral and lightly armed interposition forces following a cessation of armed hostilities, and the permission of the state on whose territory these forces are deployed, in order to discourage a renewal in military conflict and promote an environment under which the underlying dispute can be resolved."

The characteristics of this interposition force are:

After an expansion that came to include a wide range of functions and goals, today's interventions are composed of different phases and are carried out by different institutions. What can be strictly considered as the Peacekeeping part of these operations is still close to the original core mandate of traditional missions.

Peacekeeping Operations (PKOs) are not carried out exclusively by the UN, and have come to include multiple tasks, performed by military and civil personnel, single states, international and regional organizations, and NGOs. Operations start at different stages of a conflict, at times continue even through the hostilities and are protracted for a long time after a peace agreement is reached. Preventive interventions, Peacemaking and Peacebuilding can all be present and overlap as parts of a Peacekeeping Operation.

New definitions, such as in An Agenda for Peace and in the Brahimi Report on UN Peace Operations reflect the important evolution of international peace operations after the Cold War, from passive monitoring to more active engagement, but tend to include under the same denomination of Peacekeeping, missions that are very different one from another. While potentially including multiple and diverse functions such as Preventive Deployment, Peace Enforcement, Humanitarian Assistance, and Peacebuilding, the term Peacekeeping here will be used to refer to a specific function and a component of complex multitask operations.