Definition:Cartesian Coordinate System/Y Coordinate

Definition
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.

Also known as
An archaic term for the $y$ coordinate which is sometimes seen is ordinate.

Also see

 * Definition:X Coordinate