How should I signify the $y$ value of a given point

Question

This is more of a aesthetics question than maths. But if I want to use the $y$ value of a given point $P$ in an equation how to I write this? I can't really show any working since I've just googled for an answer.

Answer 1

This is commonly named the projection of $P$ over the $y$-axis, denoted by $\text{proj}_y(P)$. In the link given, note in particular the bullet set theory under the applications tab.

Answer 2

Depending on what you are using it for, you can define $P=(x,y)$ and use $y$, you can use $P_y$, you can use $\operatorname{proj}_y(P)$