"Noise Spectral Density" a.k.a "Power Spectral Density" in datasheets
Power spectral density is given in power per $Hz$. "Input Noise Spectral Density" is given in units of volts per $\sqrt{Hz}$.  These are related:

$$
PSD = \frac{W}{Hz}
$$

$$
W = \frac{V^{2}}{R}
$$

$$
Input\ noise\ spectral\ density\ = \frac{V}{\sqrt{Hz}}
$$

The $\sqrt{Hz}$ is from the conversion power to volts.  For example if you want the noise ($nV$) in $1kHz$ $BW$ you must multiply $\frac{nV}{\sqrt{Hz}}\sqrt{1000}$