Joint probability density
= Joint probability density
{title2=$f_{X,Y}(x,y)$}
A joint probability density $f_{X,Y}$ satisfies
$$
\mathbb P((X,Y)\in A)=\iint_A f_{X,Y}(x,y)\,dx\,dy.
$$
Its coordinate integrals are the <marginal distribution>[marginal densities], and integrating it over a rectangle gives the corresponding <joint distribution function>.