**(i) Parallel Lines**

If the perpendicular distance between two lines is always constant, then these are called parallel lines. In other words, the lines which never intersect each other are called parallel lines.

To define parallel lines, we must know about point, lines, and distance between the lines and the point of intersection.

(ii) Perpendicular lines

If two lines intersect each other at 90**°**, then these are called perpendicular lines. We are required to define line and the angle before defining perpendicular lines.

(iii) Line segment

A straight line drawn from any point to any other point is called as line segment. To define a line segment, we must know about point and line segment.

(iv) Radius of a circle

It is the distance between the centres of a circle to any point lying on the circle. To define the radius of a circle, we must know about point and circle.

(v) Square

A square is a quadrilateral having all sides of equal length and all angles of same measure, i.e.90**°**, To define square, we must know about quadrilateral, side, and angle.