The sentence s "In many supervised learning problems one has an output variable $y$ and a vector of input variables $x$ described via a joint probability distribution $P(x,y)$" from wiki

Here implicitly declared 3 notations $\{x,y,P(x,y)\}$.

So I wonder if there is a math notation so that we can explicitly declare some notation.

For example, we can use @, then s equals to "In many supervised learning problems one has an output variable @ $y$ and a vector of input variables @ $x$ described via a joint probability distribution @ $P(x,y)$", in my personal notebook it's very handy and clear.