Constitutional means relating to a constitution, which is a set of fundamental rules and principles that establishes how a government works and protects people's rights. When something is constitutional, it follows these rules. When something is unconstitutional, it violates them.

The United States Constitution, written in 1787, serves as the supreme law of the land. It created three branches of government (executive, legislative, and judicial) and outlined what each branch can and cannot do. The Constitution also includes the Bill of Rights, which guarantees freedoms like speech, religion, and assembly. When the Supreme Court rules that a law is constitutional, they're saying it doesn't conflict with the Constitution. When they strike down a law as unconstitutional, they're declaring it breaks these fundamental rules.

Many other countries have constitutions too. Some are short documents like America's; others fill thick books with detailed rules. A constitutional monarchy, like the United Kingdom or Japan, has a king or queen whose powers are limited by constitutional rules, unlike an absolute monarchy where the monarch's word is law.

The word can also describe something related to a person's physical or mental makeup (their constitution), but this meaning is less common today.