The amount by which the implied probabilities of all outcomes in a market add up to more than 100%, representing the bookmaker's margin.
Also called the vig, juice or hold. A three-way football market might sum to about 105%, so the extra 5% is the built-in edge the bookmaker keeps whoever wins. A lower overround means better value for the bettor, which is why comparing margins across bookmakers pays off. See our guide to how betting odds work.