"Governed by chance; not completely determined by other factors. Non-deterministic."
Note that "random" does not mean either "haphazard" or "without obvious pattern".
In practice, something is only considered "random" when any patterns that exist are either created by a "random number generator" or indistinguishable from patterns created by a random number generator.
- B. S. Everitt (2002): The Cambridge Dictionary of Statistics, Second Edition, Cambridge.