Meaning of W in SFPW, or a synonym that starts with a W

Solution 1:

W is commonly used for probability in physics. It comes, as speculated earlier, from the German language because of the high number of German physicists. For example:

http://en.wikipedia.org/wiki/Boltzmann%27s_entropy_formula

"The value of W was originally intended to be proportional to the Wahrscheinlichkeit (the German word for probability) of a macroscopic state for some probability distribution of possible microstates ..."

Solution 2:

Since Bosch Rexroth sounds decidedly German, I would think that the W stands for Wahrscheinlichkeit (German for probability), which answers your first question, but doesn't really help with your second, unfortunately.