Mediums are pieces of technology, commonly seen in the Wild Arms series, that bestows mystical powers onto their wielders. Mediums are closely related to Guardians and can be used to summon them.
In Wild Arms 2[]
In Wild Arms 2 all Mediums are named after the Guardian they are related to, and can be equipped like any other piece of equipment. Equipping it increases the character's stats and grants a special move they can use in battles by selecting the "Special" command (star icon). Both the stat increase and the move gained depends on the Medium equipped.
Tim Rhymeless needs Mediums to unlock his special attacks. He needs to defeat a certain number of enemies with a specific Medium equipped to unlock attacks associated with the corresponding Guardian. For example, he learns "First Aid" by defeating five enemies with Odoryuk Medium equipped.
Additionally, most characters can use the "Combine" Force Ability in order to summon the Guardian whose Medium they currently use.
In Wild Arms 4[]
While Mediums are not directly mentioned in Wild Arms 4, Yulie uses a special bracelet during her "Material" Force Ability, which summons a Guardian. It's possible that this bracelet is a Medium, or similar to it.
In Wild Arms 5[]
In Wild Arms 5 Mediums are plot-important items, with each Medium belonging to a specific character. Despite that, they can be freely swapped between characters. There's also Blank Mediums, which can be permanently imprinted with an original Medium. They function no different from the originals, and can be used to equip several characters with the same Medium.
Mediums play a crucial role in development of characters. Each Medium has its own GC Graph, and characters will use this GC Graph as long as they're equipped with this Medium. It also means that the player can change the party's roles in combat simply by swapping Mediums.