Definition:Cartesian Coordinate System/X Coordinate

Identification of Point in Plane with Ordered Pair
Consider a cartesian coordinate plane with an $X$-axis and a $Y$-axis.

Let a point $Q$ be positioned on the plane.

The distance of the line segment from $Q$ to the $Y$-axis is known as the $X$ coordinate and (usually) denoted $x$.

If $Q$ is to the right of the $Y$-axis, then $x$ is positive.

If $Q$ is to the left of the $Y$-axis, then $x$ is negative.

The $X$ coordinate of all points on the $Y$-axis is zero.

Also see

 * Definition:Y Coordinate