What Do We Mean by a Runaway Greenhouse Effect

What do we mean by a runaway greenhouse effect?

A) A greenhouse effect that keeps getting stronger until all of a planet's greenhouse gases are in its atmosphere
B) A greenhouse effect that starts on a planet but later disappears as gases are lost to space
C) A greenhouse effect that heats a planet so much that its surface rock melts
D) 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