Juice, also called vig or vigorish, is the margin a bookmaker builds into its prices. It is often seen in American odds, where both sides of an even contest are priced at -110 instead of +100. A bettor then has to win more than half of such bets just to break even. The percentage that must be won to break even equals the implied probability of the price. Shopping for better prices lowers the juice paid, but it is never zero in a normal bookmaker market.
Example
At -110 on both sides, a winning 110 unit bet returns 100 in profit. The implied probability is 110 / 210, or 52.38%, so a bettor must win about 52.4% of bets to break even.