CoreXY is a technique used to move the printhead of a 3D printer or the toolhead in CNC machines in the horizontal plane.
Instead, drive belts are used which are connected in an intricate way[1] to provide movement in a Cartesian coordinate system.
Compared to conventional Cartesian 3D printers for fused filament, it can provide increased printing speed due to less moving mass.
Other Cartesian 3D printers which do not use the CoreXY technique most commonly also use two motors for the xy-plane, but where one motor is independently responsible for movement along the x-axis, and the other independently responsible for movement along the y-axis.
[2] "Bed slinger" is a cartesian variant where the build surface moves along the y-axis, and the print head moves along the x-axis, and this technique is used on amongst other the Prusa i3 and clones.