NCERT Class 9 Solutions: Introduction To Euclid's Geometry (Chapter 5) Exercise 5.1 – Part 2
Q2 Give a definition for each of the following terms. Are there other terms that need to be defined first? What are they how might you define them?

parallel lines

perpendicular lines

line segment

radius of a circle

square
Solution:
To define the terms given in the question we need to define the following terms first

Point:  A small dot made by sharp pencil on a sheet paper gives an idea about a point. A point has no dimension, it has only position. Ideal point is not possible.

Line:  A line is the set of points which has length only and no breadth. The basic concept about a line is that it should be straight and that it should extend in definitely in both the directions.

Plane:  The surface of a smooth wall or the surfaces of a sheet of paper are close examples of a plane.

Ray:  A part of line l which has only one endpoint A and contains the point B is called a ray AB.

Angle:  An angle is the union o two noncollinear rays with a common initial point.

Circle:  A circle is the set of all those points in a plane whose distance from a fixed point remains constant. The fixed point is called the centre of the circle.
OA = OB = OC = radius

Quadrilateral:  A closed figure made of four line segment is called a quadrilateral

Parallel Lines:  Line which do not intersect each other anywhere are called parallel lines.
In fig m and n parallel.

Perpendicular lines:  Two lines AB and CD lying the same plane are said to be perpendicular, if they form a right angle. We write AB ┴ CD.

Linesegment:  A linesegment is a part of line. When two distinct points, say A and B on a line are given, then the part of this line with endpoints A and B is called the linesegment.
It is named as AB, AB and BA denote the same linesegment.

Radius:  The distance from the centre to a point on the circle is called the radius of the circle. In the following figure OP is the radius.

Square:  A quadrilateral in which all the four angles are right angles and four sides are equal is called a square. ABCD is a square.