Derating
Derating (or de-rating or de-tuning) is the operation of a machine at less than its rated maximum power in order to prolong its life. The term is commonly applied to electrical and electronic devices and to internal combustion engines.
In electronics
Power electronic devices have a maximum power dissipation rating usually quoted at a case temperature of 25 °C (77 °F). The datasheet for the device also includes a derating curve which indicates how much a device will dissipate without getting damaged at any given case temperature and this must be taken into account while designing a system.
As can be seen from the derating curve image for a hypothetical BJT, the device (rated for 100 W at 25 °C (77 °F)) cannot be expected to dissipate anything more than about 40 W if the ambient temperature is such that the temperature at which the device's case will stabilise (after heat-sinking) is 65 °C (149 °F). This final case temperature is a function of the thermal resistance between the device's case and the heat-sink; and the heat-sink and the ambient (this includes the heat-sink's temp/watt rating - with lower values implying better cooling characteristics).
In electrical installations
All dimmers rely on heat conduction and convection to keep the electronic components cool. Similarly, power wiring (e.g., house wiring) not surrounded by an air space (e.g., inside a conduit) needs to have its current-limiting device (e.g. circuit breaker or fuse) adjusted so as not to carry as much current through that circuit. Derating is the reduction of the maximum capacity (load) a unit can reliably handle when fins/side sections are removed.
Internal combustion engines
Internal combustion engines are normally given different ratings for different applications. For example, a diesel engine rated at 1,000 horsepower for locomotive use may be de-rated by 10% to 900 horsepower for marine or stationary use.
References
Datasheet of power transistor MJL0281A on OnSemi (PDF)