In the American political lexicon, few words are as prevalent — or as confusing — as “populism.”

Senator Elizabeth Warren (D-Mass.) gets described as a populist because she wants to curb the power of corporations and increase Social Security benefits. So does Senator Ted Cruz (R-Tex.), who thinks small businesses are crippled by “an explosion of regulation” and has called Social Security a “Ponzi scheme” that should be replaced by individual savings accounts.

Journalists, meanwhile, routinely affix the P word to liberals who want to raise taxes on the rich and to conservatives who claim higher taxes just benefit liberal special interests.

The same word is applied to office-holders whose ideologies are poles apart because populism in the United States is not a philosophical creed, like liberalism or conservatism. It is a mode of persuasion — by which left and right compete in the never-ending battle to define the virtuous many and the immoral few.

Populists on both sides view ordinary people as a noble assemblage not bounded narrowly by class or occupation; see their elite opponents — “the establishment” — as self-serving and undemocratic, and seek to mobilize the former against the latter. In a nation where the people are supposed to rule, the evolving contest to fill those vague yet emotionally powerful terms with meaning helps explain who gets to govern the nation and for what ends.