From AMS Glossary
The change in the radiative effects of clouds in response to an external climate perturbation.
The total cloud feedback is the combined result of changes in cloud cover, cloud height, and cloud reflectivity. The feedback is the net effect of these changes on the amount of sunlight absorbed by the earth and the amount of heat it radiates to space. Because many different properties of clouds contribute to cloud feedback, and because different types of clouds (e.g., cirrus vs stratus) have different effects on solar and terrestrial radiation, cloud feedback is difficult to specify with confidence. Current climate models disagree as to whether the overall cloud feedback is positive or negative. This disagreement is a major source of uncertainty in estimates of the overall sensitivity of the global climate to anthropogenic perturbations.