A "Grand Slam" in tennis refers to winning all four major tournaments in a single calendar year. These tournaments are the Australian Open, French Open, Wimbledon, and the US Open. Achieving a Grand Slam is a rare and prestigious accomplishment that highlights a player's dominance in the sport. In addition to the calendar year Grand Slam, there is also a "Career Grand Slam," which means a player has won each of the four major tournaments at least once during their career. Notable players who have achieved this include Roger Federer, Serena Williams, and Rafael Nadal.