Euclidean principles refer to the foundational concepts of geometry established by the ancient Greek mathematician Euclid. His work, particularly in the book Elements, outlines the properties and relationships of points, lines, angles, and shapes in a flat, two-dimensional space. These principles include concepts such as the parallel postulate, which states that through a point not on a line, there is exactly one line parallel to the given line. These principles form the basis for classical geometry and are used to derive various geometric theorems. They are essential for understanding shapes like triangles, circles, and polygons, and have applications in fields such as architecture, engineering, and computer graphics.