Command Meld

In Kingdom Hearts Birth by Sleep, synthesis does not occur, instead there is Command Charge. In Command Charge, you mix 2 commands to form a new command. In Command Charge, like Item Synthesis, you can add materials, namely crystals, to add effects.

Examples
For example, if you mixed Fire Blitz and Aero, you would get Aerial Slam. So if you added a Moisture Crystal to Fire Blitz and Aero, you would get an HP UP instead.