A fair bet is one for which the expected value of the payoff is zero, after accounting for the cost of the bet. For example, suppose I offer to pay you $2 if a fair coin lands heads, but you must ante up $1 to play. Your expected payoff is −$1+ $0×P(tails) + $2×P(heads) = −$1 + $2×50% = $0. This is a fair bet—in the long run, if you made this bet over and over again, you would expect to break even.