Variance
The spread of results around their expectation: how far one session's result can differ from EV purely because of the deck. Over a short distance it is bigger than the result itself.
Variance is the distance between what a decision was worth and what it brought, taken over many decisions. Aces against kings win 82% of the time, but in this hand they either win or they do not: minus $121 or plus $121, never plus $75. The gap between result and EV in one hand is variance in its purest form.
Over distance the spread averages out, but slowly. Over 300 hands a session of correct decisions can end a hundred dollars above or below its EV, and that is not a counting error, it is the norm. Over the fifteen sessions of the series the result is +$112 and the EV +$24: the deck was ninety dollars kinder to me than I earned, and not one of those dollars says anything about the quality of play.
An example from session 11
A♥A♠ against J♣J♦, all-in on the flop, my share 91.62%: of the 990 ways the last two cards can fall, I win 907. Run twice, one run-out lost and one won, result minus $1.50 against an EV of plus $117.65. A few hands later K♥Q♣ against T♠9♠ with a 25% share brought plus $98.50 against an EV of minus $49.87. Two hands, and in both the deck did the opposite of what they were worth.
The same in business
Variance in business is the reason you cannot learn from one outcome. One successful launch does not prove the method works, one failure does not prove the opposite, and whoever has understood that stops rewriting the strategy after every quarter. The only defence against variance is distance, and the only way to survive it is a stake that matters without being critical.
The whole hand is in session 11: “You will never learn whether you were right about most of your decisions”.
The other terms are in the glossary.