Control engineers would call this a runaway controller.

When the response to a change is to increase the change, rather than decrease it (which is what, for example, thermostats normally do), change happens very quickly, and uncontrollably. One of the issues with global warming is whether or not the feedback is positive, or negative. That is, does the warming result in even more warming, resulting in…or do things happen at higher temperatures that result in cooling?

One potential positive feedback might be that if glaciers and ice caps melt, the albedo of the planet decreases, which means that less energy is reflected back into space, which could result in further warming. In the other direction, if we are headed into a new glacial period despite the greenhouse effects (perhaps because solar activity dominates all else), then increasing snow cover makes things colder because more solar energy is rereflected, thus causing more cooling, and an accelerating glacial advance.

On the negative feedback side, though, it could be that more warming results in more clouds, which might in turn have the effect of cooling things off.

I suspect that the reality will be a combination of positive and negative feedback mechanisms, and it’s hard to know what the overall effect will be, though ultimately, it will be negative, but perhaps at a significantly higher (or lower) temperature. I’d be very surprised if the seas end up either boiling, or freezing solid.