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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.
Parallel lines:
Two or more lines which never intersect at any point are called parallel lines. These lines always has a constant perpendicular distance.
![parallel Ресурс 1.png](https://resources.cdn.yaclass.in/ba3809fe-c639-4d91-a83c-1b4d2343244e/parallel%D0%A0%D0%B5%D1%81%D1%83%D1%80%D1%811w300png.png)
Perpendicular lines:
Two lines are said to be perpendicular if they intersect each other at right angle.
![perpendicular Ресурс 1.png](https://resources.cdn.yaclass.in/7e97b2eb-9a8e-4348-a74e-2c8f4670fe93/perpendicular%D0%A0%D0%B5%D1%81%D1%83%D1%80%D1%811w300png.png)
Line segment:
A line ending at two distinct points is called a line segment. A line segment has \(2\) definite endpoints.
![line segment Ресурс 2.png](https://resources.cdn.yaclass.in/7bc039b0-f822-4ff5-93fc-7fe40f633dc0/linesegment%D0%A0%D0%B5%D1%81%D1%83%D1%80%D1%812w300png.png)
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.
![radius Ресурс 1.png](https://resources.cdn.yaclass.in/2bd903a4-c71a-4a02-8920-83bf3989fe5a/radius%D0%A0%D0%B5%D1%81%D1%83%D1%80%D1%811w300png.png)
Square:
A square is a quadrilateral made by four equal straight lines. All the angles of a square are right angles.
![square1 Ресурс 1.png](https://resources.cdn.yaclass.in/41c74508-b35d-4532-bea8-01bdeab12187/square1%D0%A0%D0%B5%D1%81%D1%83%D1%80%D1%811w200png.png)
Angle:
An angle is a shape formed by two rays meeting at a common endpoint.
![angle Ресурс 1.png](https://resources.cdn.yaclass.in/4e1f8775-3cb2-41f6-82cc-c8f87476cd12/angle%D0%A0%D0%B5%D1%81%D1%83%D1%80%D1%811w300png.png)
Reference:
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.