The ATP Tour 250 tournaments (previously known as the ATP World Tour 250 tournaments, ATP International Series, and ATP World Series) are the lowest tier of annual men's tennis tournaments on the main ATP Tour, after the four Grand Slam tournaments, ATP Finals, ATP Tour Masters 1000 tournaments, and ATP Tour 500 tournaments.. As of 2019, the series includes 39 tournaments, with 250 ranking ...
https://en.wikipedia.org/wiki/ATP_World_Tour_250_series