1.
a. : a state characterized by the actual existence of open armed hostilities (as between nations) regardless of a formal declaration of war by any party to the conflict
b. : a legal state that comes into being by formal declaration regardless of whether open armed hostilities have taken place, must be officially proclaimed at an end by a similar declaration, and is usually characterized by such conditions that the rights and duties of belligerents and neutrals to act under the rules of international law applicable to war arise
2. : the period of time during which a state of war is in effect