Definition:Probability

Context
Probability Theory.

Informal Definition
The probability of an event is the likelihood that the event will occur (within a certain formally or informally understood context).

As such, a probability can range from $$0$$ (will never happen) to $$1$$ (is certain to happen).

Commonly, a probability is often stated in terms of a percentage.

Formal Definition
Let $$\mathcal E$$ be an experiment.

Let $$A \in \Sigma$$ be an event in the event space $$\Sigma$$ of $$\mathcal E$$.

Let $$\Pr$$ be the probability measure of $$\mathcal E$$.

Then the probability of $$A$$ is the value $$\Pr \left({A}\right)$$.

Comment
Amongst the non-mathematically-literate of the world's population, a common form of rhetorical emphasis (usually used when asserting a falsehood of a personal and delicate nature) is to state a greater-than-$$100\%$$ probability of an event.

A typical example of this could be seen on the Maury Povich Show, as follows:
 * Euniquea: "I am five hundred thousand percent certain that Twyvone is the father of mah bay-bee ..."
 * Maury Povich: "Twyvone ... you are naaht the father."