To most peopls, "probability" is a loosely defined term employed in everyday conversation to indicate the measure of one's belief in the occurrence of a future event.
We accept this as a meaningful and practical interpretation of the term but seek a clearer understanding of the context in which it is used, how it is measured, and how probability assists in making inferences.
The concept of probability is necessary when dealing with physical, biological, or social mechanisms that generate observations that cannot be predicted with certainty.
For example, the blood pressure of a human at a given point in time cannot be predicted with certaincy, and we never know the exact load that a bridge will endure before collapsing inte a river.
Such random events cannot be prediced with certainty, but the relative frequency with which they occur in a long series of trials is often remarkably stable. Events possessing this property are called
random or
stochastic events.
Source - Mendenhall Scheaffer Wackerly.