Thermal noise

In telecommunication, thermal noise is the noise generated by thermal agitation of electrons in a conductor. The noise power, P , in watts, is given by P = kT Δf , where k is Boltzmann's constant in joules per kelvin, T is the conductor temperature in kelvins, and Δf is the bandwidth in hertz.

Note 1: Thermal noise power, per hertz, is equal throughout the frequency spectrum, depending only on k and T .

Note 2: For the general case, the above definition may be held to apply to charge carriers in any type of conducting medium. Synonym Johnson noise.

Source: from Federal Standard 1037C and from MIL-STD-188