Regarding N1 and N2, what does the "N" stand for? I initially phrased this question in a comment to another question regarding what N1 and N2 mean, hoping it would be that sort of low-hanging fruit that is easily answerable and not worth a proper question in its own right.  But then I starting searching and came up short.  

This Yahoo Answers page has lots of guesses on it, from "nominal" to "Newell".  (And we all now are reminded why nobody goes to that site for real answers.)  Many other pages explain what N1 and N2 are, but not where the "N" part comes from.

I'm wondering if it might have its origin in German, as a lot of early developmental work on jet engines was done in Germany.  Or was it simply an unused letter that fit?
As far as I can tell, the N comes from the physics behind turbines in general, whether they be water pumps or wind turbines, a subset of mathematics called fluid dynamics.  Many thanks to ROIMaison's answer for steering me in the right direction.

In Turbomachinery: Basic Theory and Applications, Second Edition, Earl Logan, Jr. provides a symbol reference list at the end of each chapter starting in chapter 2, defining $N$ as rotor speed, in units of rad/s; an angular change per unit time, and $N_s$ as the specific speed, a dimension-less quantity derived from a ratio of the flow and head coefficients.



N1 and N2 seem to be derived from both.  Wikipedia mentions the speed as a percentage paradigm as a human factors consideration, though as of 5/14/2015 there are no references to support this.  However, there may be a scientific basis behind this as well, since a change in N is proportional to the resultant change in airflow (i.e. thrust produced):