A plane is considered an undefined term in geometry primarily because it:
is described generally, not using a formal definition.
While it can be named using three noncollinear points, and it serves as the foundation for constructing other geometric shapes, the essence of being an "undefined term" lies in the fact that it doesn't have a precise, formal definition beyond its general description as a flat surface extending indefinitely in all directions. The undefined terms in geometry (like point, line, and plane) are fundamental concepts that we understand intuitively but do not define with other terms.