Derating
Derating means using a component at less than its maximum rating because the operating conditions are harsher than the conditions it was rated under. Heat is the most common reason, since a part that is already hot cannot safely dissipate as much additional power. Manufacturers publish derating curves that show the allowed power or current at each temperature.
Example: A resistor rated for 300 watts on a cool heatsink may only be safe at 75 watts when its mounting surface is held at 150 degrees Celsius.