It's basically a catch-all term that applies to wide variety of political figures from William Jennings Bryan to Juan Peron, Adolf Hitler, Bernie Sanders, and Donald Trump.
I think it's supposed to mean any ideology that attacks the establishment and appeals to lower class people. The people it appeals to could be farmers, miners, the urban poor, union workers, etc. Of course, every populist movement attracts wealthy people and business interests (Silver mining companies for WJB, celebrities who supported Sanders, and the various business interests and media personalities that supported Trump). It has more to do with a candidate's style than their policies.
In that case, wouldn't a successful third party almost have to be populist, by it's very nature?
I don't think so. You could have a Gary Johnson-Lincoln Chafee-Susan Collins-Michael Bloomberg-Charlie Baker elitist soft-libertarian third party. The "socially liberal, but fiscally conservative" people.
I don't think they would ever control much, though. They would just screw Democrats out of much of the Northeast. If we're talking about a 3rd party that can actually win statewide elections anywhere, an elite-libertarian coalition simply hasn't worked.
I think the most likely 3rd party movement going forward is some kind of Mormon/Texas HFC-style coalition, particularly if Trump eventually goes FDR 1938 on the HFC and tries to primary them all out.
But again, you probably just end up screwing your favorite major party by doing this. The HFC strikes me as anti-establishment enough to truly not care, though.
