A useful WashPo review of Jan-Werner Müller’s book on populism quotes the three defining features identified by Müller:

First, populists are anti-elitists, meaning they criticize the established political, cultural and economic leadership. Second, they must be anti-pluralist, claiming sole representation of the people. When Trump says that “I alone can fix” what ails us, or assures supporters that “I am your voice,” he is asserting uncontested, unmediated leadership. Finally, populism is exclusionary, in the sense that “the people” are an increasingly circumscribed set; though they might begin as the white working class or another loosely defined group, they are quickly reduced to supporters of the leader. Otherwise, you are traitorous, inauthentic. “This is the core claim of populism,” Müller writes. “Only some of the people are really the people.”