A plane is an undefined term because it

can be named using three noncollinear points

is described generally, not using a formal definition

is where other geometric shapes can be constructed

is a flat surface that extends indefinitely in all directions

Question

A plane is an undefined term because it 

 can be named using three noncollinear points 

 is described generally, not using a formal definition 

 is where other geometric shapes can be constructed 

 is a flat surface that extends indefinitely in all directions

directrix · Answer

Point, line, and plane are the three undefined terms in classical Euclidean Geometry.

There is no formal definition for plane.