kT is the product of the Boltzmann constant, k, and the temperature, T. This product is used in physics as a scaling factor for energy values in molecular-scale systems (sometimes it is used as a unit of energy), as the rates and frequencies of many processes and phenomena depend not on their energy alone, but on the ratio of that energy and kT, that is, on E / kT (see Arrhenius equation, Boltzmann factor). For a system in equilibrium in canonical ensemble, the probability of the system being in energy E is given by e-ΔE / kT. More fundamentally, kT is the amount of heat required to increase the thermodynamic entropy of a system, in natural units, by one nat.
In Chemistry and Materials Science, the kinetics of a reaction or a phase transformation often depends on the activation energy Q, the configurational energy difference between reactant,intermediate or final molecular/crystal structures. At temperature T, kT is the energy per molecule that is spontaneously available in the system in the form of its degrees of freedom (translational, vibrational and rotational). If kT is larger than the barrier itself, the reaction/transformation becomes essentially barrier-less at the molecular level and proceeds spontaneously.
In macroscopic scale systems, with large numbers of molecules, RT value is commonly used; its SI units are joules per mole (J/mol): (RT = kT * NA). At room temperature 25 °C (77 °F, 298 K) 1kT is equivalent to 4.11x10−21 J , 4.11 pN·nm , 9.83x10−22 cal, 0.0256 eV, 2.479 kJ·mol−1 or 0.593 kcal·mol−1.