The temperature at which a standard sample of a material will deflect 0.010 inches under a specified load, in this case 264 pounds per square inch. (Also know as heat-distortion point). Typically, a material with a higher HDT will perform better, that is, resist distortion better, in high-heat situations. However, an evaluation of the exact environment and potential stresses of the end use is recommended for the final determination of material suitability.

See Deflection temperature under load (DTUL).

The overall definition of heat deflection temperature is a temperature at which a polymer sample deflects a certain amount under heat and load. There is not good consistency between test methods, making data sheet reading difficult. The test can be conducted on three different thicknesses, two different loading conditions, and yet the specific test is rarely called out. What follows is one test definition: The temperature where the material shows a deformation of .01 inches under a constant load (66 or 264 psi). The heat deflection temperature is a function of the temperature, strain rate, and stress.