What is Constitutional Amendment?

Legal Definition
A constitutional amendment refers to the modification of the constitution of a nation or state. In many jurisdictions the text of the constitution itself is altered; in others the text is not changed, but the amendments change its effect. The method of modification is typically written into the constitution itself.

Most constitutions require that amendments cannot be enacted unless they have passed a special procedure that is more stringent than that required of ordinary legislation. Examples of such special procedures include supermajorities in the legislature, or direct approval by the electorate in a referendum, or even a combination of two or more different special procedures. A referendum to amend the constitution may also be triggered in some jurisdictions by popular initiative.

Australia and Ireland provide examples of constitutions requiring that all amendments are first passed by the legislature before being submitted to the people; in the case of Ireland, a simple majority of those voting at the electorate is all that is required, whereas a more complex set of criteria must be met in Australia (a majority of voters in a majority of states is also necessary). Switzerland has procedure similar to that of Australia.

The special procedures for the amendment of some constitutions have proven to be so exacting that of proposed amendments either few (8 Amendments out of 44 proposed), as in Australia, or none, as in Japan, have been passed over a period of several decades. In contrast, the constitution of the U.S. state of Alabama has been amended over 800 times since 1901.
-- Wikipedia
Legal Definition
A formal revision or addition proposed or made to the US Constitution. Article V of the Constitution details how amendments are made; there are several methods to propose an amendment, ratification requires the approval of 3/4ths of the states. There are 27 amendments to the Constitution.