I had to miss class today due to another engagement and in learning what I missed on Bivariate Probability Distributions I've come across this notation.

$\displaystyle f_{XY}(x,y)$

Is this another notation for a function of two variables or does it denote partial derivatives?

$\displaystyle f_{XY}(x,y)=f(x,y)$?

or

$\displaystyle f_{XY}(x,y)=\frac{d}{dx}\frac{d}{dy}f(x,y)$?

Or something else?

Thanks for the clarification!

Kasper