This is a good, brief article by Conor Lynch on "populism." Our Pod Pundits call anything that's designed to appeal to a voter who's not a One Percenter "populism." At least based on this article, even the Big Dawg himself (Bill Clinton) is a bit confused on the subject. In European politics, populism is generally used to refer to rightwing demagoguery.
He also quotes Jim Hightower, who was the campaign manager for Democratic Sen. Fred Harris' New Populist campaign for the Democratic Presidential nomination in 1976. I like his politics. But I don't buy his definition of populism as a “historically grounded political doctrine that supports ordinary folks in their ongoing democratic struggle for power over their lives." I agree with Lynch that there's nothing inherently "left" or "right" about populism. A populist movement can be one or the other. Or even an unholy mixture of both.
Lynch doesn't quote the late Argentine political theorist Ernesto Laclau on the topic. But the definition he uses is consistent. "Narrative" is a favorite academic word these days. And Laclau describes populism as a narrative strategy for constructing a political identity for the "people" in opposition to the "elite," the classic populist dualism.
According to that definition, populism is an approach to defining a political contest in terms of the People vs. the Elite. It can be put to service by various ideologies, from militant social democracy to political Islam to rightwing xenophobia.
Laclau has several useful concepts in defining the process, like the "empty signifier," an image or concept that takes on a newly charged political meaning that can unite the People being constructed. Occupy Wall Street gave us a great example of that. Prior to 2011, the phrase One Percent wouldn't have had any particular significance for most people. Now it's even used in business publications, everybody knows what it means, and it's generally understood in the sense of the Elite that is the standard enemy of populism.