Give a definition for each of the following terms. Are there other terms that need to be defined first? What are they, and how might you define them?.

(1) Parallel lines: Two lines, having no common points, are parallel lines. [Undefined term: The distance between the two lines are constant]
(i) Perpendicular lines: Two lines which form angle of 90° with each other, are perpendicular lines.
(iii) Line segment: The line which have two end point, is a line segment.
(iv) Radius of a Circle: The line segment from the centre of circle to the circle is radius of circle.
(v) Square: A quadrilateral having all sides equal and all angles right angle.