Definition:Cartesian Plane/Ordered Pair

Identification of Point in Plane with Ordered Pair
Let $Q$ be a point on the Cartesian plane.

Construct two straight lines through $Q$:
 * one perpendicular to the $x$-axis, intersecting the $x$-axis at the point $x$
 * one perpendicular to the $y$-axis, intersecting the $y$-axis at the point $y$.

The point $Q$ is then uniquely identified by the ordered pair $\tuple {x, y}$.

Also known as
The ordered pair $\tuple {x, y}$ which determine the location of $P$ in the cartesian plane can be referred to as the rectangular coordinates or (commonly) just coordinates of $P$.