Vig (hold)
Vig is the margin a sportsbook builds into its prices, which is why the implied probabilities of both sides of a market add up to more than 100 percent.
In plain English
Also called juice or hold. If both sides of a coin flip were priced fairly they would sit at +100 each. Pricing them at -110 each is the book taking a cut, and that cut is the hurdle every bettor pays before skill matters.
Example
Both sides at -110 imply about 52.4 percent each, totalling 104.8 percent. The excess is the vig.
Why it matters
Vig is the reason betting is not a fair game by default. Beating it, not merely picking winners, is what a positive record requires.