Definition:Axis/X-Axis

Definition
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.

Also see

 * Definition:Y-Axis
 * Definition:Z-Axis