It is defined as the time required for a thermistor to change 63.2% of the total difference between its initial and final body temperature when subjected to a step function change in temperature The thermal time constant is affected by the medium in which the test is performed. For example, the thermal time constant will be shorter in moving air than in still air and shorter in moving water than in still water $\tau_{th}=\frac{\rho V c_p}{hA_s}$ $V$ is volume of the object $c_p$ is specific heat $A_s$ is surface area Units of time constant is is seconds