Euclidean Definitions

A line has length between two points but no width.

A point has no components i.e. it can’t be divided into parts.

A straight line traces the closest distance between two points.

A curved line has no straight segments.

A surface or a plane is  a completeky flat surface between its border lines. Lines constitutes the borders of planes.

Two straight lines are said to intersect each other in a point where they meet. The inclination between those lines is called the angle between the lines.

The point of intersection is called the vertex of the angle.

A circle is a planar figure limited by a curved line called the perimeter or periphery.

The distance from the center to the perimeter is called the radius and is everywhere on the circle  the same.

The diameter is a straight line from periphery to peiriphery through the center of the circle. It divides the circle into two equal parts-half-circles.

A triangle is a figure made up of three straight lines. These are called its  sides.

A quadrilateral figure is made up of four straight lines.

A polygonal figure is borders by more than four straight lines.

A triangle composed of three equal sides is called equilateral.

A triangle having two equal sides is termed isosceles triangle.

A right triangle has a perpendicular angle.