Definition for Runaway greenhouse effect
From Biology Forums Dictionary
The runaway greenhouse effect is an unstable situation that occurs when an atmosphere changes its composition in a way that leads it to continue to change more and more rapidly. As an example, water vapor in the atmosphere is a greenhouse gas: its presence traps infrared radiation and increases the temperature of an atmosphere. Now if the amount of water vapor were to increase, perhaps by volcanic outgassing, then the temperature would increase.