Thermal Noise

Johnson–Nyquist noise (thermal noise, Johnson noise, or Nyquist noise) is the electronic noise generated by the thermal agitation of the charge carriers (usually the electrons) inside an electrical conductor at equilibrium, which happens regardless of any applied voltage.Thermal noise is approximately white, meaning that the power spectral density is equal throughout the frequency spectrum. Additionally, the amplitude of the signal has very nearly a Gaussian probability density function.
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The noise generated by thermal agitation of electrons in a conductor. The noise power, P , in watts, is given by P = kT f , where k is Boltzmann's constant in joules per kelvin, T is the conductor temperature in kelvins, and f is the bandwidth in hertz. (188 ) 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.