Vig (vigorish)
The bookmaker's built-in commission, baked into the odds of every market.
Vig — also called juice or the margin — is the cut a bookmaker takes by pricing a market so the odds imply more than 100% total probability. On a fair coin toss paying 1.91 each way, the vig is about 4.5%. Use the margin calculator to strip it out and see the true price.