Definitions of Technical Terms

Power rating (W): The maximum continuous power that may be dissipated by a resistor whose ohmic value is less than or equal to the Critical Resistance and at or below the Rated Ambient Temperature. Also known as Rated Power. In the case of arrays and networks, the rating given here is per resistor element; see datasheet for package rating.
Rated ambient temperature (°C): The highest ambient temperature at which the full Rated Power may be dissipated. Ambient temperature is defined as the temperature which would be adopted by the body of the resistor under application conditions except with negligible power dissipation in the resistor. For through-hole packages this is usually air temperature, for heatsink mounted packages it is heatsink temperature, and for SMD packages it is local PCB temperature.
Derate to 0W temperature (°C): The temperature at which the derating curve meets zero watts. This normally equals the Maximum Ambient Temperature, but for some power resistors, may exceed it. Derating is normally linear from full Rated Power at Rated Ambient Temperature, unless the datasheet states otherwise.
Ambient temperature, maximum (°C): The maximum ambient temperature which the component is designed to withstand for continuous operation. Also known as Upper Category Temperature (UCT). (See Rated Ambient Temperature for a definition of ambient temperature.)
Limiting element voltage (V): The maximum continuous voltage which may be applied across a resistor whose ohmic value is greater than or equal to the critical resistance. Also known as Maximum Working Voltage. Usually it is DC or AC rms, but the datasheet for high voltage parts may define it as DC or AC peak.
Resistance, minimum (Ω): The minimum ohmic value available in the standard range.
Resistance, maximum (Ω): The maximum ohmic value available in the standard range.
Critical resistance (Ω): The resistance value at which application of the Limiting Element Voltage causes dissipation of the full Rated Power. At ohmic values below this, the Rated Power is the limiting factor which restricts electrical loading, whilst at ohmic values above this, the Limiting Element Voltage is the limiting factor.
Best Tolerance (± %): The highest precision grade of tolerance offered, which may apply to a restricted ohmic value range. Tolerance is defined as the maximum allowable magnitude of deviation from nominal ohmic value measured at delivery and at room temperature. In the case of arrays and networks, the tolerance given here is ratio tolerance; see datasheet for absolute tolerance.
Best TCR (± ppm/ °C): The highest precision grade of TCR offered, which may apply to a restricted ohmic value range. TCR (Temperature Coefficient of Resistance) is defined as the maximum magnitude of reversible ohmic value change per degree centigrade due to deviation in body temperature from a reference normally of 20°C. In the case of arrays and networks, the TCR given here is tracking TCR; see datasheet for absolute TCR.
Isolation voltage (V): The maximum continuous voltage which may be applied between the resistor and a conductor in contact with its insulated body.