Cas Mudde and Cristobal Rovira Kaltwasser's Populism: A Very Short Introduction:
"we define populism as a thin-centered ideology that considers society to be ultimately separated into two homogeneous and antagonistic camps, "the pure people" versus "the corrupt elite," and which argues that politics should be an expression of the volonté générale (general will) of the people." (kl 700-705)
"Pluralism is the direct opposite of the dualist perspective of both populism and elitism, instead holding that society is divided into a broad variety of partly overlapping social groups with different ideas and interests. Within pluralism diversity is seen as a strength rather than a weakness. Pluralists believe that a society should have many centers of power and that politics, through compromise and consensus, should reflect the interests and values of as many different groups as possible. Thus, the main idea is that power is supposed to be distributed throughout society in order to avoid specific groups— be they men; ethnic communities; economic, intellectual, military or political cadres, etc.— acquiring the capacity to impose their will upon the others." (kl 733-738).
h/t Dan Little