I have stumbled twice across this symbol
|| being used in the analysis of transistor circuits.
An example can be seen at this address: http://www.learningaboutelectronics.com/Articles/Dc-analysis-of-a-bipolar-junction-transistor-circuit
I can understand it:
x||y = (xy)/(x+y)
But what is it called and why is it used? Because I do not know the name, I was unable to find anything with a search. "Double vertical line transistor circuit analysis" didn't return any results.