I am reading Properties of sample variance, and suddenly this notation comes up with no introductory explanation what it means

as in

since

and

(when proving that the sample variance is an unbiased estimator of the population variance)

Then the next chapter said 'the sample mean and the sample variance are independent,

(the Y was supposed to be sample mean, ie with a bar on top, but I couldn't find the way how to place the bar in Latex?)

So my guess is that this sign in statistical inference denotes mutual independence, but I'd like to check.

Thanks!

PS found this in Google

http://www.psych.umn.edu/faculty/wal...gs/rodgers.pdf
apparently, (linearly) independent is not the same as orthogonal or uncorrelated. So what exactly does that 'perpendicular' sign denotes?