Euclid defined \(23\) definitions in his book \(1\) "Elements". Some of them are listed below.
1. A point is that which has no part.
2. A line is breadthless length.
3. The ends of a line are points.
4. A straight line is a line which lies evenly with the points on itself.
5. A surface is that which has length and breadth only.
6. The edges of a surface are lines.
7. A plane surface is a surface which lies evenly with the straight lines on itself.
Few more definitions of the words, which we used in geometry are given below.
Two or more lines which never intersect at any point are called parallel lines. These lines always has a constant perpendicular distance.
Two lines are said to be perpendicular if they intersect each other at right angle.
A line ending at two distinct points is called a line segment. A line segment has \(2\) definite endpoints.
Radius of a circle:
A line segment drawn from centre of the circle to the circumference of the circle is called a radius of the circle.
A square is a quadrilateral made by four equal straight lines. All the angles of a square are right angles.
An angle is a shape formed by two rays meeting at a common endpoint.
National Council of Educational Research and Training (2006). Mathematics. Chapter-5 Introduction to Euclid's Geometry(pp. 78-88). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.