Slam Dunk is the fourth ball that is unlocked, that can be found in Sensei's rack in the Ninja Kit. Slam Dunk is just after the Red Star. Slam Dunk has an appearance of a basketball. This basketball (ball) earns 14,000 XP and 500 coins and is unlocked at level 24. The Slam Dunk shares the same physical features as a basketball.
It is most likely called Slam Dunk because it resembles a basketball, and in basketball, you can slam dunk.