Why was "tantrum" trending last week?
1818, as a term in church administration, from plural + -ism. Attested from 1882 as a term in philosophy for a theory which recognizes more than one ultimate principle. In political science, attested from 1919 (in Harold J. Laski) in sense "theory which opposes monolithic state power." General sense of "toleration of diversity within a society or state" is from 1933. Related: Pluralist (1620s, in the church sense); pluralistic.
A conviction that various religious, ethnic, racial, and political groups should be allowed to thrive in a single society. In metaphysics, pluralism can also mean an alternative to dualism and monism. A pluralist asserts that there are more than two kinds of principles, whereas the dualist maintains there are only two and a monist only one.