If you write about democratic theory, as I do, you will periodically get complaints that it is inaccurate to refer to the United States as a “democracy” because it is actually a “republic.” For example, several Facebook commenters and others have suggested that I should have titled my book Democracy and Political Ignorance (which focuses primarily on political ignorance in the United States) “The Republic and Political Ignorance” or something to that effect.
In the 18th century, “democracy” and “republic” were relatively distinct terms, with the former referring mainly to what we would today call “direct democracy,” of the sort practiced by the ancient Athenians. But today, the word “democracy” is routinely used to describe any government where all or most political leaders are chosen by popular election. Moreover, governments are regularly described as “democratic” even if they have a variety of constraints on the powers of elected officials, such as federalism, separation of powers, judicial review, and so on. By this definition, the United States surely qualifies as a democracy, even if it can also be called a “republic.” The two terms have become largely interchangeable, with the exception of the fact that a democracy that has a figurehead constitutional monarch as head of state will usually not be called a republic.
This is not a recent innovation. The terms were often used interchangeably, including in reference to the United States, by the mid-19th century. For example, Abraham Lincoln described the United States as “a constitutional republic, or a democracy—a government of the people, by the same people” in this 1861 message to Congress.
People who insist on a sharp distinction between “republic” and “democracy” may simply dislike modern usage and prefer a return to that of two hundred years ago. But if so, they should not claim – as they often do – that those who use the words in their modern sense are ignorant or incorrect. In this case, as in most others, correctness is determined primarily by usage rather than some “objective” definition of linguistic propriety. As used by most English-speakers today, “republic” and “democracy” are largely interchangeable terms.
Moreover, the whole republic vs. democracy debate is a rhetorical distraction from the substantive point that should really concern us: what constraints should there be on the power of voters and the officials they elect? That issue cannot be resolved by claiming that we are a “republic” rather than a “democracy.” Even if there is a substantive distinction between the two and the United States is clearly a republic and not a democracy, that says nothing about whether the constraints we currently impose on majoritarian institutions are too strong, too weak, or roughly correct.
UPDATE: Eugene Volokh made some related points in this 2010 post.