Vig
Vig (vigorish) is the sportsbook margin built into odds. It makes implied probabilities sum to more than 1 and reduces expected value for bettors.
Definition
Vig (short for vigorish) is the margin sportsbooks include in quoted odds. It is another way to describe overround.
Why it matters
When you convert odds into implied probabilities, the vig means those probabilities are inflated. If you want “fair” probabilities for comparison to your forecasts, you typically remove the vig.
Related
See removing the vig for normalization, and overround for the aggregate margin concept.