Answer:
A promise by the United States to keep peace to the American continents.
Better answer:
The Monroe Doctrine was a statement of U.S. foreign policy issued by President James Monroe in 1823. It declared that the Western Hemisphere was no longer open for colonization or territorial expansion by European powers, and that any attempt by a European power to interfere in the affairs of the newly independent nations of the Americas would be considered a threat to the security and interests of the United States. The doctrine was intended to assert U.S. leadership and influence in the Western Hemisphere, and to prevent European intervention in the region.