The word "progressive" has a very specific meaning for historians, but in its contemporary use, it's just a synonym for liberal, at least that's how I use it. For a writer it's just easier to cut out repetition by using more than one word for the same thing. Chris Moody at the Daily Caller has a pretty funny piece on just how meaningless the distinction between "progressive" and "liberal" actually is.
“I'm not sure what the definition is,” conceded James Rucker, executive director of Color of Change. “I don’t love the term.” Rucker co-founded his organization with former White House “green jobs czar” Van Jones, so there isn’t much question about where he stands politically. But the term still strikes him as opaque. “I think it’s kind of the new ‘liberal,’” he said.
Ambiguous? That may be the point. “People use the word ‘progressive' these days in part because the word liberal has been discredited by the right,” said Roger Hickey, co-director of Campaign for America’s Future, a Washington-based non-profit that touts itself as “the strategy center for the progressive movement.”
Maybe that's it. But the flip side of this is that conservatives aren't any less confused. They've spent the last few years coming up with hilariously convoluted explanations for how contemporary progressives represent a more sinister, extreme form of liberalism, when they're basically aware that the desire to avoid the stigma of "liberal" is the most likely explanation for the term coming into vogue. It doesn't matter what term liberals use, conservatives committed to the idea that liberal participation in politics is inherently illegitimate will always develop an internal political mythology that justifies their anti-liberal eliminationism.