TERM
Value bet
A bet on an outcome whose odds are higher than fair, meaning the bookmaker underestimates its probability; unlike a surebet, you can still lose.
A value bet is a single bet, not a set. Its value lies in positive expected value over the long run: if the fair odds are 2.00 and a book offers 2.20, each such bet returns 10% of turnover on average, but any particular bet can lose.
Fair odds are usually taken from the sharpest book in the market or averaged across several. A surebet is a special case where there's value on both outcomes at once at different books, and the risk is covered completely.