Climate sensitivity is the degree to which global climate responds to a change in the amount of energy in the climate system. More specifically it is defined as the "equilibrium change in response to changes of the radiative forcing."
In terms of anthropogenic climate change, it indicates how much the climate will change in response to human factors affecting climate, such as CO2 emissions, land use changes and deforestation - or forcings. Lower sensitivity means global climate change is relatively little in response to these forcings. Higher sensitivity means a greater response.
As the evidence continues to mount on the urgency of dealing with human-induced global climate change, climate sensitivity has become a last bastion for climate deniers, claiming that even if humans are causing global warming, due to the low sensitivity of the climate, it really won't make that much difference, at least until far into the future.
Often sensitivity is only considered in terms of amount of CO2 in the atmosphere, but, as the following video describes, ongoing research indicates a much more complex picture of how climate sensitivity works. This research continues to debunk the idea that climate sensitivity is too low for real concern or action.
Image credit: Crowbared, courtesy flickr