A word coined by former Silicon Alley Reporter columnist Clay Shirky to describe the free time that people have on their hands to engage in collaborative activities, specially as applies to web 2.0.
Wikipedia is an example of wide-scale deployment of cognitive surplus.

"You know where the time comes from. It comes from the cognitive surplus that you've been masking for 50 years".
by Arunabh Das Two May 25, 2008
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