Definition:X-Y Plane

Definition
The $x$-$y$ plane is the Cartesian plane embedded in Cartesian $3$-space which contains the $x$-axis and the $y$-axis.

It consists of all the points in $S$ such that $z = 0$.

Also see

 * Definition:$y$-$z$ plane
 * Definition:$x$-$z$ plane