"Cost Sharing" occurs when the University "shares" a portion of a sponsored project's costs. Cost sharing can take many forms, including payroll, equipment costs, Facilities and Administrative costs, etc. For effort reporting purposes, cost sharing is effort committed in a proposal where all or part of the funding for that effort is not provided by the sponsor. In such a case, the percentage of salary charged to a sponsored project is less than the percentage of effort devoted to that project; the difference represents cost sharing.
For additional information, please see the Columbia University Policy on Cost Sharing and the Cost Sharing Reference Guide (pdf).