That's a term fondly used by Russian ultra-nationalists, the sort of people who'd be called fascists if they lived in any other country in Europe.
They're going to Odessa. That's the goal. The problem with this is that once they cross from Donetsk to the rest of Ukraine, they no longer even have the fig leaf of a "referendum" to justify the action. Of course by now it's pretty obvious what this is, so I guess that really doesn't matter anymore.
Between Transnistria, which they are looking to hook up to, and Georgia and their little "republics", it's pretty obvious his long term goal is the reimposition of Russian rule over as much of the old USSR as he can get away with. Some student asked him about the allegedly rough treatment Russians got in Kazakhstan today, so I guess they should start watching their borders a little more closely too.
The word for this is "revanchism". See below:
Revanchism (from French: revanche, "revenge"

is a term used since the 1870s to describe a political manifestation of the will to reverse territorial losses incurred by a country, often following a war or social movement. Revanchism draws its strength from patriotic and retributionist thought and is often motivated by economic or geo-political factors. Extreme revanchist ideologues often represent a hawkish stance, suggesting that desired objectives can be achieved through the positive outcome of another war.
Revanchism is linked with irredentism, the conception that a part of the cultural and ethnic nation remains "unredeemed" outside the borders of its appropriate nation-state. Revanchist politics often rely on the identification of a nation with a nation-state, often mobilizing deep-rooted sentiments of ethnic nationalism, claiming territories outside of the state where members of the ethnic group live, while using heavy-handed nationalism to mobilize support for these aims. Revanchist justifications are often presented as based on ancient or even autochthonous occupation of a territory since "time immemorial", an assertion that is usually inextricably involved in revanchism and irredentism, justifying them in the eyes of their proponents.
Link:
http://en.wikipedia.org/wiki/Revanchism
What Putin is doing exactly follows that definition.
I've read a lot of Marx, Engels, Lenin, and others, and I don't recall the above being a part of what they were advocating. I might be mistaken on that, of course, but I don't think so.