Here's another term you might hear around equestrian sport.
There are two things that are generally called Derbys:
1. A major and significant horse race, usually for colts or colts and geldings. The Kentucky Derby, Epsom Derby, Irish Derby, etc, are amongst the most prestigious races in the world.
2. A major show jumping contest, almost always held outside. The Hickstead Derby is the most famous example. It is known for being contested over the exact same course every single year, including several permanent obstacles.