The Constitution is a document, written in 1787 and amended several times since, that serves to define the system of government of the United States of America.

The Constitution of the United States (sometimes written as the United States Constitution or just the Constitution) is always capitalized because it is the title of a document. It is the supreme law of the United States, and it lays out the framework for the organization of the federal government, including its legal authority and its relationship with the states, the citizens and everyone else in the country.
The U.S. Constitution was written in 1787, when delegates from twelve of the thirteen states convened in Philadelphia, originally, to revise the Articles of Confederation (the first constitution of the United States). Through discussion and debate, they finally came to the decision that they needed more than a revision; they needed an entirely new constitution. The debates and the writing of the document were completed on September 17, 1787.
The Constitution was originally made up of a preamble and seven articles, but it has since come to include 27 amendments as well. The preamble to the Constitution is as follows:
“We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity, do ordain and establish this Constitution for the United States of America.”
A few years after the original articles of the Constitution were ratified, ten amendments were added to clarify some specific freedoms of U.S. citizens. These ten amendments are called the Bill of Rights. Since they were ratified, 17 more changes have been made to the Constitution to further define our rights, powers and limits as citizens.
The preamble, the seven original articles, the Bill of Rights and the 17 additional amendments, all together, constitute the Constitution.