"Reversion to the mean is the statistical phenomenon stating that the greater the deviation of a random variate from its mean, the greater the probability that the next measured variate will deviate less far. In other words, an extreme event is likely to be followed by a less extreme event."

My friend insists there is no requirement for randomness and uses a similar definition of mean reversion but without the word "random". The application is using mean reversion for the pricing of commodities.

So, is randomness a necessary condition for mean reversion?

Is there a better definition of mean reversion?

How can I convince my friend that mean reversion only applies to random variables?

Thanks in advance.