Definition:Cartesian Coordinate System/Z Coordinate

Definition
Consider a cartesian $3$-space with an $x$-axis, a $y$-axis and a $z$-axis.

Let a point $Q$ be positioned in space.

The distance of the line segment from $Q$ to the $x$-$y$ plane is known as the $z$ coordinate and (usually) denoted $z$.

If $Q$ is above the $x$-$y$ plane, then $z$ is positive.

If $Q$ is below the $x$-$y$ plane, then $z$ is negative.

The $z$ coordinate of all points on the $x$-$y$ plane is zero.

Also see

 * Definition:X Coordinate
 * Definition:Y Coordinate