In tennis, a Grand Slam is when a player wins all four major tournaments in one year. These tournaments are the Australian Open, French Open, Wimbledon, and the US Open.
Each tournament is played on a different surface and in a different country. Winning a Grand Slam is extremely difficult and rare in tennis.
Only five singles players have ever achieved a Grand Slam.
The last man to do it was Rod Laver in 1969. Steffi Graf is the only woman to win a Grand Slam in the Open Era. She did it in 1988. Many great players like Roger Federer and Serena Williams never won a Grand Slam.
The term is also commonly used for these four tournaments collectively as "Grand Slam Tournaments" or simply "The Majors."