A little bit of this, a little bit of that....
Mix (ちょうごう, Chōgō?) is a recurring ability throughout the Final Fantasy series. It is normally associated with the Chemist class. Mix fuses two items to create a more powerful one. Usually it is used right away, but sometimes it can be stored for later use.
Mix is the Level 2 ability for the Chemists learned for 30 AP.
The Alchemist's Smelt command lets the player combine items to create new ones.
Compounding is a Medication ability learned by Salve-Makers at job level 1. It combines two items for a special effect.