Derate:  A decrease in the available capacity of an electric generating unit, commonly due to:

• A system or equipment modification
• Environmental, operational, or reliability considerations. Causes of generator capacity deratings include high cooling water temperatures, equipment degradation, and historical performance during peak demand periods. In this context, a derate is typically temporary and due to transient conditions.

The term derate can also refer to discounting a portion of a generating units capacity for planning purposes.