In George Tsebelis's theory, an individual or collective actor whose agreement is required to change the policy status quo.
In more detail
Institutional veto players are set by the constitution, such as a second chamber or a president. Partisan veto players arise from parties, such as coalition partners. Tsebelis argued that policy stability rises with the number of veto players, the ideological distance between them and their internal cohesion. The theory predicts how hard change is, not whether the resulting policies are good.