Wiktionary
n. (context US English) A place where a gambler can bet on various sports competitions.
Wikipedia
In the United States a sportsbook or a race and sports book (sometimes abbreviated as book) is a place where a gambler can wager on various sports competitions, including golf, football, basketball, baseball, hockey, soccer, horse racing, boxing, and mixed martial arts. The method of betting varies with the sport and the type of game. In the US, the Professional and Amateur Sports Protection Act of 1992 allows only Nevada, Oregon, Montana, and Delaware to legally wager on sports.
Winning bets are paid when the event finishes, or if not finished, when played long enough to become official; otherwise all bets are returned. This policy can cause some confusion since there can be a difference between what the sportsbook considers official and what the sports league consider official. Customers should carefully read the sportsbook rules before placing their bets.
The betting volume at sportsbooks varies throughout the year. Bettors have more interest in certain types of sports and increase the money wagered when those sports are in season. Major sporting events that don't follow a specific schedule, like boxing, can create peaks of activity for the sportsbooks.