What is Commander-in-chief?

Legal Definition
A commander-in-chief is the person or body that exercises supreme operational command and control of a nation's military forces or significant elements of those forces. In the latter case, the force element is those forces within a particular region, or associated by function. As a practical term; it refers to military competencies that reside in a nation-state's executive leadership—either a head of state, a head of government, a minister of defence, a national cabinet, or some other collegial body. Often, a given country's commander-in-chief (if held by an official) need not be or have been a commissioned officer or even a veteran. This follows the principle of civilian control of the military.
-- Wikipedia
Legal Definition
By article 2, § 2, of the constitution it is declared that the president shall be commander in chief of the army and navy of the United States. The term implies supreme control of military operations during the progress of a war, not only on the side of strategy and tactics, but also in reference to the political and international aspects of the war. See Fleming v. Page, 9 How. 603, 13 L. Ed. 276; Prize Cases, 2 Black, 635, 17 L. Ed. 459; Swaim v. U. S., 28 Ct. CI. 173.
-- Black's Law Dictionary