\begin{definition}[Definition:Axis/X-Axis]
In a cartesian coordinate system, the '''$x$-axis''' is the one usually depicted and visualised as going from left to right.
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 $x$ are zero.
\end{definition}