Populist translates as a politician who tells the people what they want to hear. Of course every politician tells people what they want to hear. You won't win elections by insulting your audience and telling them that you will increase taxes, raise the retirement age and offer nothing in return.
What discerns discerns populists in the usual definition from other politicians is opportunism. "Normal" politicians follow to a considerable extent their convictions. They may discard some of their more extreme beliefs as unfeasible and they may highlight the attractive parts of their plans while hiding the less attractive parts, but for the most part they follow their convictions.
Populists on the other hand have few fixed ideas about what they want to do. They will promise people whatever they want to hear. Only when they are in power will they bother to look how to implement their ideas. Many of their promises will vaporize at the moment they are chosen. As long as they deliver a few and organize good pr around it their supporters will be happy.
Trump is a typical populist. Very likely he wouldn't care about a wall at all if it wasn't so popular with his audience. His promises about infrastructure have long been forgotten. And his trade wars to protect America's industry have become a bit messy as he is shooting from the hip and has no idea what how such things work.
Sanders on the other hand has a clear idea what he wants. His ideas are not mainstream and if he might rise to power he certainly would encounter situations that he hadn't thought through. But his promises derive from his ideas about society and not from opportunism. So it would be wrong to call him a populist.