Golden Sun: The Lost Age - Psynergy Guide
"Psynergy", is the magic or psychic power that manipulates the forces of nature. Beings who harness this power are called "Adepts". Adepts are able to use this power to perform a number of actions that aren't originally possible.
Psynergy specifically for battle are extremely useful. There are also field psynergy which can be used to solve many puzzles that appear in your way.
On your quest, you will find psynergy gems which bestows special psynergy to its holder. All, if not most, are designed to aid you while solving puzzles.