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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.
Perpendicular lines:
Two lines are said to be perpendicular if they intersect each other at right angle.
Line segment:
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.
Square:
A square is a quadrilateral made by four equal straight lines. All the angles of a square are right angles.
Angle:
An angle is a shape formed by two rays meeting at a common endpoint.
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.
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