Definition:Cartesian Coordinate System/Y 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 $X$-axis is known as the $Y$ coordinate and (usually) denoted $y$.

If $Q$ is above the $X$-axis, then $y$ is positive.

If $Q$ is below the $X$-axis, then $y$ is negative.

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

Also see

 * Definition:X Coordinate