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