Solution 9 (ソリューション9, Soryūshon 9?) is a recurring ability in the series. It is a Limit Break-esque ability used by Zidane Tribal. It may be named so as an allusion to the number nine as it originates from Final Fantasy IX.
IX Solution 9 is an ability used by Zidane only while in Trance and while having access to the ability Lucky Seven. It inflicts massive non-elemental damage to one enemy equal to 300% of Zidane's regular attack at the cost of 48 MP.
Dissidia Solution 9 is an aerial Bravery magic attack used by Zidane, learned at level 24. It requires 140 AP to master and 30 CP to equip (15 when mastered), has Ranged Low priority, and is capable of Feint.
Dissidia012 Solution 9 returns as a Bravery attack for Zidane, learned at level 23. It now costs 100 AP to master but is otherwise the same as before.
The ability is likely named thusly as an allusion to number 9, a recurring element in Final Fantasy IX.