X

X is the most commonly used for FREE/ independent variable in math, if we say y=f(x) it's mean y is function of x, or y depend on x,....
uh oh..... too complicated to xplain...
the point is.....
I want to be free....
and I want to have some effects to .....
U

I hope it can be U = f(X) :P
it good meaning of course... :)

please continue to read ...
U'll find Why XYZ ? :D

...... curious?