In communications, **noise spectral density** (**NSD**), **noise power density**, **noise power spectral density**, or simply **noise density** (*N*_{0}) is the power spectral density of noise or the noise power per unit of bandwidth. It has dimension of power over frequency, whose SI unit is watt per hertz (equivalent to watt-second or joule).
It is commonly used in link budgets as the denominator of the important figure-of-merit ratios, such as carrier-to-noise-density ratio as well as *E*_{b}/*N*_{0} and *E*_{s}/*N*_{0}.

If the noise is one-sided white noise, i.e., constant with frequency, then the total noise power *N* integrated over a bandwidth *B* is *N* = *BN*_{0} (for double-sided white noise, the bandwidth is doubled, so *N* is *BN*_{0}/2). This is utilized in signal-to-noise ratio calculations.

For thermal noise, its spectral density is given by *N*_{0} = *kT*, where *k* is the Boltzmann constant in joules per kelvin, and *T* is the receiver system noise temperature in kelvins.

The **noise amplitude spectral density** is the square root of the noise power spectral density, and is given in units such as .^{[1]}^{[2]}