From the Kingdom Hearts Wiki: A world of information not accessible by Gummiship
Ether Boost (エーテルアップ, Ēteru Appu?, lit. "Ether Up") is an ability that appears in Kingdom Hearts 358/2 Days. It increases the number of magic casts recovered by using Ethers, Hi-Ethers, and Mega-Ethers by one.
In Kingdom Hearts 358/2 Days, Ether Boost is an equipment ability.
Learning Ether Boost
Kingdom Hearts 358/2 Days
Rings with Ether Boost