Oxford English Dictionary 4. The presence or tolerance of a diversity of ethnic or cultural groups within a society or state; (the advocacy of) toleration or acceptance of the coexistence of differing views, values, cultures, etc.
Pluralism applies differently to politics, culture, religion, and philosophy, but overall pluralism is the theory or attitude that . . .
No single voice or text embodies all the truth in every condition.
Instead, truth is more or less reached or realized through expression and interaction of varying viewpoints.
Most thinkers naturally think they're right and that anyone who disagrees is wrong . . .
but experience (usually in the form of being wrong, or other people continuing to disagree even when you're right) teaches that . . .
large-scale society functions better not when one person or group imposes its views on others, but when a variety of viewpoints is expressed and developed—the wrong parts of any viewpoint get worked out, while the better parts may be optimized.
If society represses legitimate viewpoints or positions, those repressed viewpoints often express themselves in more dangerous or violent ways. For instance, freedom of religion: if religion is repressed, religion can turn nasty and vengeful.
As Thomas Jefferson wrote,
"The legitimate powers of government extend to such acts only as are injurious to others. But it does me no injury for my neighbour to say there are twenty gods, or no god. It neither picks my pocket nor breaks my leg. If it be said, his testimony in a court of justice cannot be relied on, reject it then, and be the stigma on him. Constraint may make him worse by making him a hypocrite, but it will never make him a truer man. It may fix him obstinately in his errors, but will not cure them."
American politics' experience with pluralism is mixed, partly because in democracy a majority wins.
The most consistently pluralistic strain in American politics is its commitment to multi-party elections, though usually there are only two parties.
Another expression of pluralism is through political support and lobbying by interest groups, though extreme wealth-inequality has converted this to "pay-to-play" politics.
"Pluralism is the theory that a multitude of groups [e.g. "interest groups" or factions], not the people as a whole, govern the United States. These organizations, which include among others unions, trade and professional associations, environmentalists, civil rights activists, business and financial lobbies, and formal and informal coalitions of like-minded citizens, influence the making and administration of laws and policy." (http://www.socialstudieshelp.com/APGOV_pluralism.htm)