What Is Noise?
Noise is anything that obscures the desired signal. Usually, it is random noise of a physical, often thermal, origin. The thermal noise of an object is the measure of amount of energy in each of its degrees of freedom. For example, the output signal from an amplifier is a degree of freedom, so it has a minimum energy, or noise, due to its temperature. Thermal noise is approximately 1.27 x 10^-20 Watts/Hertz. Thernal noise power is determined by the equation
Pnoise= kTB where k is Boltzmann's constant, T is the absolute temperature (Kelvin), and B is the bandwidth in Hertz.
Electronic equipment will also create noise by itself. This is because electric current is the flow of discrete electric charges, not a constant fluid flow. So the number of electrons passing a specific point in some amount of time will vary some amount from moment to moment. This statistical fluctuation of the current is called shot noise.