The document regarded as the cornerstone of the U.S. Constitution is the "Preamble." The Preamble sets the stage for the Constitution by outlining the fundamental purposes and guiding principles of the government. It begins with the famous phrase "We the People," emphasizing that the authority of the government derives from the people. The Preamble lists the goals of the Constitution, including establishing justice, ensuring domestic tranquility, providing for the common defense, promoting the general welfare, and securing the blessings of liberty. It serves as a succinct summary of the Constitution’s intent and is often referenced in legal and political discussions.