Variance
Variance is the natural spread of outcomes around a long-run average.
In plain English
Even a genuinely profitable approach loses for stretches, and a losing one wins for stretches. Variance is why a short run of results cannot distinguish the two, and why judging a process by its last ten bets is unreliable in both directions.
Example
A method that wins 55 percent of the time can easily lose eight of twelve without anything having changed.
Why it matters
Most bad betting decisions come from reading variance as information, then reacting to it.