A feedback is something that amplifies or attenuates an initial effect (e.g. compound interest is a feedback on a loan). A feedback that takes place within hours or years is called a fast feedback, whereas one that takes over decades or centuries is called a slow feedback. In climate science, water vapor and clouds cause fast feedbacks, whereas melting ice sheets cause slow feedbacks.
1. The 2008 study Target atmospheric CO2: Where should humanity aim? led by James Hansen claimed that climate sensitivity to "fast feedback processes" is 3°C, but when accounting for longer-term feedbacks (such as ice sheet disintegration, vegetation migration, and greenhouse gas release from soils, tundra or ocean), if atmospheric CO2 remains at the doubled level, the sensitivity increases to 6°C based on paleoclimatic data.