The Lesson
A probability is a measure of how likely (how probable) an event is to happen. A probability is expressed as a number between 0 (impossible) and 1 (certain).Dictionary Definition
The Oxford English Dictionary defines a probability as "the extent to which a particular event is likely to occur... expressed by a number between 0 and 1.The Probability Scale
Probabilities express how likely an event is to happen on a scale of 0 to 1.- An event with a probability of 0 is impossible: it can not happen.
- An event with possibility of 1 is certain: it will happen.

Expressing Probabilities
A probability can be expressed as:- a number between 0 and 1.
- a fraction.
- a percentage between 0% and 100%.

A Note on Notation
The probability of an event can be written as:
P(Event)
A letter or symbol can be used to represent an event.
For example, let H be the event that a coin lands on Heads when it has been tossed.
We can denote the probability of getting heads as:
P(H)