An amendment in a legal context refers to a change made by a legislative or parliamentary process. The founding fathers devised a dual process by which the U.S. Constitution could be changed.
The first option must begin in Congress which, by a two-thirds vote (of a quorum) in each house, may initiate an amendment. Alternatively, the legislatures of two-thirds of the states may ask Congress to call a national convention to discuss and draft amendments.
To date, all amendments have been proposed by Congress; although state legislatures have on occasion requested the calling of a convention, no such request has yet received the required concurrence of two-thirds of the states.
In either case, amendments must have the approval of the legislatures or conventions of three-fourths of the existing states before they become part of the Constitution. The Constitution has a total of 27 amendments. However, since the first ten of the amendments, collectively known as the Bill of Rights, were ratified simultaneously, it has actually only been amended 18 times.