In Europa Universalis II, a white peace is a form of peace treaty which is imposed automatically when there are no provinces occupied and there has been no active fighting (which includes battles, seiges and assaults, but not cover) for a period of 36 consecutive months. The state or war simply ends, with no gains or losses to either war coalition. The aggressor loses the war, in the few cases which that matters.
White peace may also be proposed as a peace treaty in which no demands or offers are made. Often, AIs will propose white peace if the warscore is near zero and they don't border you.