When Benjamin Franklin walked out of the Constitutional Convention, a woman asked him what they had made. A republic, he said, if you can keep it.
A republic is a government where power belongs to the people and is exercised through elected representatives, not held by a monarch and not decided by the raw show of hands of the whole crowd. The word comes from the Latin res publica, the public thing, the common business of the people.
The framers chose it deliberately. They feared kings, but they also feared pure direct democracy, which they worried could become mob rule, a majority trampling a minority on the passion of a single day. A republic, with representatives and written limits, was meant to slow that down.
The famous story comes from 1787. As Franklin left the convention, Elizabeth Powel of Philadelphia is said to have asked him what kind of government they had created. His reply has echoed ever since: a republic, if you can keep it. Not a gift, but a responsibility.
That last clause is the whole point. A republic is not self-sustaining. It depends on citizens who show up, who vote, who serve on juries, who hold power to account. Let them stop, and the thing quietly stops being a republic at all.