Euclidean refers to the geometry based on the work of the ancient Greek mathematician Euclid. His most famous work, "Elements," outlines the fundamental principles of geometry, including points, lines, angles, and shapes. In this system, the familiar rules of geometry, such as the Pythagorean theorem, apply to flat surfaces. In Euclidean geometry, space is considered two-dimensional or three-dimensional, where the concepts of distance and angle are consistent. This contrasts with non-Euclidean geometries, which explore curved spaces and different postulates. Euclidean geometry remains foundational in mathematics, architecture, and various fields of science.