Boltzmann Probability
When a thermodynamic system is in contact with a heat reservoir, heat transfer may occur between the system and the reservoir. As a result of processes of this type, the energy of the system fluctuates with time. The Boltzmann probability addresses the question: "If the reservoir has a Kelvin temperature, T, what is the probability of the system having an energy, E?" Statistical thermodynamics provides the answer: P(E)T = exp ( - E/kT) , where k is Boltzmann's constant (8.6 x 10-5eV/K).