In tennis, a Default means a player is disqualified from a match by the umpire.
It happens when a player gets too many code violations for bad behavior. The violations could be for yelling, breaking a racket, or arguing with officials.
Defaults are rare, but some top players have been defaulted, like Novak Djokovic. In 2020, Djokovic accidentally hit a line judge with a ball and was defaulted.
John McEnroe was famously defaulted from the 1990 Australian Open for misconduct and arguing.
If both players in a match are defaulted, it's called a "double default."