In the case of statistics, there are two terms: "Variance" and "Standard Deviation" to - for in the latter case - have one to note the case in which the units are without squares.
Am I correct to understand, that there is no such differentiation in signal processing, and in both cases - taken root or not - it's always termed "Power Spectral Density"?
I am asking as on page 3 of Analog Devices AD8307 there's the unit of $\frac{nV}{\sqrt{Hz}}$ for the "Input Noise Spectral Density".
Is there a specific reason as to why manufacturers are taking the square-root?