The definition of probability
The probability of an event is the number of successful outcomes
divided by the number of possible outcomes (Laplace, 18th century).
A more serious definitions:
Experiment --- a procedure that yields one of one of a given set of
possible outcomes (e.g., flipping the coin).
The sample space of the experiment is the set S of possible outcomes
An event is a subset E of the sample space (e.g. {head}).
The probability of E, denoted by p(E), is |E|/|S|.