EV (Expected Value)
The average amount you win or lose per identical decision over the long run.
Expected value is the heart of every profitable poker decision. Mathematically, EV = sum of (probability × outcome) for every possible result of a play. A call has positive EV when your equity in the pot is greater than the price you're paying for the call.
Players obsess over results, but results are noisy. EV is signal. A call that loses today but is +EV every time is the call you keep making. The pros call this "playing the hand the same way every time, regardless of outcome".
Practical use: before any non-trivial call or bet, ask "what's the long-run profit if I face this exact spot 1,000 times?" That number is your EV — and it's the only one that matters.