In the mind. A plane is an abstract concept: it is perfectly even (flat) and infinitely long in every direction. A flat surface, such a s desktop or sheet of paper may model it, but these will only be approximations.
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Two dimensional geometry is called "plane geometry" meaning that it occurs on a single surface or plane. The objects used in plane geometry are called plane figures.
Presumably it is simply geometry. However, plane geometry is geometry which is limited to objects with up to two dimensions, that is, objects that lie in a plane - or on a flat surface.
Point ; Line ; Plane - Remember the Point-Line-Plane Postulate
The geometry of similarity in the Euclidean plane or Euclidean space.