\begin{definition}[Definition:Axis/Y-Axis]
In a cartesian coordinate system, the '''$y$-axis''' is the one usually depicted and visualised as going from "bottom" to "top" of the paper (or screen).
It consists of all the points in the real vector space in question (usually either $\R^2$ or $\R^3$) at which all the elements of its coordinates but $y$ are zero.
\end{definition}