In Kingdom Hearts, Synthesis is a procudure that is done by Moogles in the Traverse Town Workshop. The Moogle Synthesis Workshop is located behind the Traverse Town Accessory Shop. Synthesis requires items, and from it, you can synthesize different items to creative a result (Usually An Item). However, to start synthesizing you have to have all the correct items and the amount of items needed. To Synthesize talk to the Main Moogle in the Shop, he will direct you to the creation of the Synthesis-created item, it will indicate on the screen whether or not you can create the item, and it will also display your current inventory of that item.

Creating a series of items will allow you to create more items in a different series.