onstitutes a "full" or "partial" meltdown—neither is a strictly technical term, though in the popular usage of the word, today's revelation leans more towards the former. David Brenner, director of the Columbia University Center for Radiological Research, describes a "partial meltdown" as "fuel that's been damaged and partially melted. Some of the fuel has probably been oxidized and breached and melted at the top of the core where the heat rises."
MIT nuclear scientist Ron Ballinger describes a "full meltdown" as when fuel "would melt into the bottom of the vessel. Then you get to this theoretical point where if they can't cool it, then eventually the vessel itself, the steel, would melt, and you'd end up with a bunch of melted fuel and steel on the bottom of the concrete faceplate of the plant, in the containment vessel. And then it would have to get out of there."
Today's news indicates something much closer to the latter scenario.