### 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.

Perpendicular lines:

Two lines are said to be perpendicular if they intersect each other at right angle$\left(90\mathrm{°}\right)$.

Line segment:

A line ending at two distinct points is called a line segment. A line segment has $$2$$ definite endpoints.

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.