We have to define ‘Ray’, ‘Straight line’ and a ‘point’.

Ray: A part of a line, which starts at a point and goes off in a particular direction to infinity possibly through a second point.

Straight line: The basic concept about a line is that it should be straight, and it can be extended infinitely in both directions.

Point: A small dot made by a sharp pencil on a sheet of paper gives an idea about a point.

i) Parallel lines:

• If the perpendicular distance between two lines is always constant, then these are called parallel.
• If the lengths of the common perpendiculars at different points on the lines are the same, then these lines are called parallel.
• In other words, the lines which never intersect each other are called parallel lines.

ii) Perpendicular lines: If the angle between two lines is equal to 90º, then these lines are perpendicular to each other.

iii) Line segment: A terminated line is called a line segment. It has two endpoints.

iv) Radius of a circle: The distance from the center to any point on the circle is called the radius of the circle.

v) Square: A square is a regular quadrilateral which means that it has four equal sides and four right angles.