In geometry, what is a flat surface that extends infinitely in all directions called?

A flat surface that extends infinitely in all directions is called a plane. In geometry, a plane is defined as a two-dimensional surface with no thickness and is characterized by the fact that it stretches out infinitely in every direction. This concept is foundational in geometry, as planes are essential for understanding shapes, angles, and other geometric principles.

While the other options represent different geometric concepts, they do not describe a flat surface that extends infinitely. A line, for example, is a one-dimensional figure that extends infinitely in two directions, while a point represents a location in space and has no dimensions at all. A solid refers to a three-dimensional object that occupies space. Therefore, the definition and properties of a plane align perfectly with the characteristics described in the question.