It has dimension of power over frequency, whose SI unit is watt per hertz (W/Hz), equivalent to watt-second (W ⋅ s) or joule (J).
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 Eb/N0 and Es/N0.
This is utilized in signal-to-noise ratio calculations.
For thermal noise, its spectral density is given by N0 = kT, where k is the Boltzmann constant in joules per kelvin (J/K), 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 volts per square root of hertz,