History isn't "what actually happened", history is an argument, based on evidence, fitting into a certain theoretical framework about what happened in the past. As new evidence is discovered (or old evidence discredited) and theories change, history has to be revised.

That's the neutral version of it. In practice a lot of revisionist history is blatantly politically motivated (e.g. Holocaust denial).

Modern history is necessarily revisionist because we've been writing about it, academically and professionally, for over one hundred years. A lot has been said and all theories are somewhat incestuous. If you want to say something new it will revise what someone else has said already.

Revisionist history is basically what is sounds like: an attempt to look at events or people in a new way.

History often gets stuck in a rut when certain arguments or narratives become canonized in popular culture or in text books.

Think about Nero. He murdered Christians, killed his own mother, fiddled while Rome burned. Apparently he was a crazed megalomaniac eager for all the pleasures his wealth and power could grant him.

Except maybe instead of believing all these stories the way they are presented by ancient sources (most likely affected by the attempts of his successors to make him look as bad as possible) we should approach these stories by asking "what reasons did Nero have for doing these based on what we know about the nature of his position?" Is there a way to rehabilitate him into at least a sane megalomaniac? Many people think so (he made the cover of Nat Geo two months ago for this).

Revisionists get a bad rap in most part because some try to be shocking instead of focusing on the craft. People also don't like being told that what they think is wrong, nor are they comfortable with the idea that facts can change. Nero can go from a crazy fornicator to a shrewd operator.

Nero jumped to mind because there are two recent biographies of him the old school version is Griffin's Nero: The End of a Dynasty the revisionist Champlin's Nero.

Follow on question, why do some of my professors refer to someone as a revisionist in a seeming effort to qualify anything that person might have said, but at other times will introduce someone as a revisionist in seeming praise? Can the term be used in both a positive and negative connotation?

It just means "new(ish) historical arguments that are challenging some pre-existing and largely popular historical argument." I say "new-ish" because they aren't always as new as people realize (they can be decades old). It is usually meant to be pejorative — you're meant to think that some new theory is bad because it is new and not as established.

I think it is a terrible term to use. Historical arguments are either good or bad, persuasive or not, biased or not. Old arguments can be good, old arguments can be bad. New arguments can be good, new arguments can be bad. And a lot of what people think of today as "old" arguments are not even that old and established themselves.

Historical arguments should be evaluated on their merit, not whether they are new or old or challenging or accepted. Imagine how stupid it would sound if new scientific theories were denounced as "revisionist" because they differed from ones in the past.