\begin{definition}[Definition:Cartesian Coordinate System/X Coordinate]
Consider a Cartesian coordinate system $C$ with an $x$-axis.
Let a point $Q$ be positioned in $C$.
Let $x$ be the length of the line segment from the origin $O$ to the foot of the perpendicular from $Q$ to the $x$-axis.
Then $x$ is known as the '''$x$ coordinate'''.
If $Q$ is in the positive direction along the real number line that is the $x$-axis, then $x$ is positive.
If $Q$ is in the negative direction along the real number line that is the $x$-axis, then $x$ is negative.
\end{definition}