Definition:Event/Occurrence/Union

Definition

Let the probability space of an experiment $\EE$ be $\struct {\Omega, \Sigma, \Pr}$.

Let $A, B \in \Sigma$ be events, so that $A \subseteq \Omega$ and $B \subseteq \Omega$.

Let the outcome of the experiment be $\omega \in \Omega$.

Let $\omega \in A \cup B$, where $A \cup B$ denotes the union of $A$ and $B$.

Then either $A$ or $B$ occur.

Consider the experiment $\EE$ such that $2$ (positive) integers are drawn at random from a table of random numbers.

Let $A$ be the event that at least $1$ of these integers is prime.

Let $B$ be the event that at least $1$ of these integers is even.

Then their union $A \cup B$ means:

either of the $2$ integers is even or either of the $2$ integers is prime.

Consider the experiment $\EE$ such that $3$ devices are checked as to whether they are operational.

Let $A$ be the event that at least $1$ of these $3$ devices is defective.

Let $B$ be the event that all $3$ devices are sound.

Then their union $A \cup B$ is a certainty.

1965: A.M. Arthurs: Probability Theory ... (previous) ... (next): Chapter $2$: Probability and Discrete Sample Spaces: $2.2$ Sample spaces and events
1968: A.A. Sveshnikov: Problems in Probability Theory, Mathematical Statistics and Theory of Random Functions (translated by Richard A. Silverman) ... (previous) ... (next): $\text I$: Random Events: $1$. Relations among Random Events
1986: Geoffrey Grimmett and Dominic Welsh: Probability: An Introduction ... (previous) ... (next): $1$: Events and probabilities: $1.2$: Outcomes and events