Intel (from Sandy Bridge TMDG Rev 1.1): Analysis indicates that real applications are unlikely to cause the processor to consume maximum power dissipation for sustained time periods. Intel recommends that complete thermal solution designs target the Thermal Design Power (TDP), instead of the maximum processor power consumption.
AMD (from AMD Family 10h Desktop Processor Power and Thermal Data Sheet): The processor thermal solution should be designed to accommodate thermal design power (TDP) at Tcase Max. TDP is measured under the conditions of all cores operating at CPU COF, Tcase Max, and VDD at the voltage requested by the processor. TDP includes all power dissipated on-die from VDD, VDDNB, VDDIO, VLDT, VTT, and VDDA. TDP is not the maximum power of the processor.