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An event \(\; E = \{ w_i \} \;\) is a set of outcomes to which a probability is assigned.
An event occurs when one of its outcomes occurs.
The probability of an event is the sum of probability of its outcomes.
NOTE Each element of a sigma algebra in a probability space is an event.
1 https://stats.stackexchange.com/questions/143459/are-the-terms-event-and-outcome-synonymous
2 https://en.wikipedia.org/wiki/Event_(probability_theory)