Vig (vigorish, juice, overround, hold)
The margin a sportsbook builds into its prices: the amount by which the implied probabilities of all outcomes exceed 100%.
A fair coin would be priced +100 / +100. Books quote −110 / −110 instead, which implies 52.4% + 52.4% = 104.8%. The extra 4.8% is the overround, and the book's expected take on balanced action is that divided by the total — about 4.5%. Different books, sports and market types carry different vig, and it moves over time.
Vig is the single biggest determinant of whether cross-book arbitrage can exist: two books with 4.5% margins each need to disagree by more than the combined vig before an arb appears. ArbMonster measures the actual vig of 20+ US books from live lines, by sport and market, every scan.
Odds −110 and −110 → decimal 1.909 and 1.909 → implied 52.38% each → sum 104.76% → overround 4.76%, hold ≈ 4.5%.
Measured on ArbMonster: Measured vig for every sportsbook · Vig calculator
Related terms: Implied probability · No-vig odds · Hold · H2H