What Do We Mean by a Runaway Greenhouse Effect

What do we mean by a runaway greenhouse effect?

A) a greenhouse effect that starts on a planet but later disappears as gases are lost to space
B) a greenhouse effect that heats a planet so much that its surface rock melts
C) a process that heats a planet like a greenhouse effect,but that involves a completely different mechanism of heating that doesn't actually involve greenhouse gases
D) a greenhouse effect that keeps getting stronger until all of a planet's greenhouse gases are in its atmosphere