ATP Tour Masters 1000 – Wikipedia

The ATP Masters tournaments (previously known as ATP Masters Series) is a series of nine tennis tournaments featuring the top-ranked players on the ATP Tour.The series is the most prestigious in men’s tennis after the four Grand Slams. The series’ events are held annually in Europe, North America and Asia since the debut of ATP Tour in 1990.

https://en.wikipedia.org/wiki/ATP_Tour_Masters_1000