Theory:

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
 
Perpendicular lines:
 
Two lines are said to be perpendicular if they intersect each other at right angle(90°).
 
perpendicular  Ресурс 1.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
 
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
 
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
 
Angle:
 
An angle is a shape formed by two rays meeting at a common endpoint.
 
angle Ресурс 1.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.