Expected value (EV)
Expected value, or EV, is the average amount a decision wins or loses if it were repeated many times, found by weighting each possible outcome by how often it happens.
Expected value is the average result of a decision made over and over. You find it by weighting each possible outcome by how often it happens, then adding the results together.
EV of a call = (win% x amount won) - (lose% x amount called)
Worked example. You can call 40 to win a pot of 160, and your hand is good 50% of the time:
EV = (0.5 x 160) - (0.5 x 40) = 60
Calling here is worth 60 on average, a clear profit, even though you still lose the other half of the time.
That last point is the one that trips people up. EV is a long-run average, not a promise about the next hand: a positive EV call can lose, and a negative EV call can win, on any single try. Judge the decision by what it earns over many repetitions, not by how the one hand in front of you turns out.
Every other idea in poker, from pot odds to bluffing frequency, exists only to help find the higher EV option in a given spot. Learn to see decisions in EV terms and the rest of strategy becomes a set of tools for reaching the same answer faster.