Let $(X, \mathbb{X})$ is a measure space. The function $f:X\rightarrow \mathbb{R}$ is measurable if $$\{x\in X:f(x)>y\}\in \mathbb{X}$$ for all $y\in \mathbb{R}.$
I am familiar with this. But in a book (about analysis) I read about a jointly measurable function. There is not a definition. And I can not find any definition for this in Google.
Question: What is it? Why we need it? What were we missing in the concept of measurable functions so that we want to introduce a new concept?
EDIT: To be more precise I write the first sentence in the book where this term appears: "For later use we now show that $\chi_{\{f>t\}}$ is a jointly measurable function of a and t". What is it?