# Basic terms and definitions

In this section, let me introduce certain basic terms and definitions that would help you to understand probability better.

Let me start with Sets, sub-sets and their relevance to understanding probability better.

A set is a collection well defined objects. ‘Well defined’ implies that the definition of the object should be clear so that anyone can say that the object belongs to the given set or does not belong to the given set. Once a set is defined, if we are unable to decide whether a given object belongs to the set or not, then it serves no purpose except leading to confusion. Examples of sets can be

1. Set of all fruits. Here, the qualifier is that the object must be a fruit. If it is a fruit, it belongs to the set. Otherwise, it does not belong to the set.
2. Set of all students. Here, the qualifier is that the person concerned should be a student.

The set of all possible outcomes of a random experiment is defined as the sample space and is denoted by ‘S’. Every possible subset of ‘S’ is defined as an event. Events are denoted by A,B,C,D etc. Since the empty set or the null set is also a subset of the sample space ‘S’, it is also an event. The null set is known as the impossible event and is denoted by 