A declaration of war is a formal declaration issued by a national government indicating that a state of war

War

War is a state of organized, armed, and often prolonged conflict carried on between states, nations, or other parties typified by extreme aggression, social disruption, and usually high mortality. War should be understood as an actual, intentional and widespread armed conflict between political...

 exists between that nation and another. For the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, Article One, Section Eight of the Constitution says "Congress

United States Congress

The United States Congress is the bicameral legislature of the federal government of the United States, consisting of the Senate and the House of Representatives. The Congress meets in the United States Capitol in Washington, D.C....

 shall have power to ... declare War". However, that passage provides no specific format for what form legislation must have in order to be considered a "Declaration of War

Declaration of war

A declaration of war is a formal act by which one nation goes to war against another. The declaration is a performative speech act by an authorized party of a national government in order to create a state of war between two or more states.The legality of who is competent to declare war varies...

" nor does the Constitution itself use this term.