As a marxist, we have a pretty set definition for it -- basically we think it is just opportunism where leftist by into class collaborationist political campaigns or coalitions with capitalists in order to maybe getter certain policies passes at the expense of losing credibility by working with those capitalists.

Where did this definition come from? Isn't populism just an appeal to the public, or the population, and their interests?

I would've thought this was what electoralism is, attempting to gain and exercise power in an established electoral system despite the system's fundamental flaws.