From Ardrana

Elementalism is a magical school of effect. It is an area of specialization focusing on spells involving the four prime elements of air, earth, fire, and water. These spells may be from any of the nine traditional schools of magic. The fireball spell, for example, belongs to the invocation school, but according to elementalists, it is also a spell of elemental fire.

Each element has a diametrical opposite: air opposes earth, fire opposes water, and vice versa. Every elementalist must choose one element as his specialty. He may learn and cast any spells relating to his chosen element and gains advantages when doing so. He may also cast spells of the two elements which do not oppose his specialty, for which he receives no bonuses or penalties. Consequently, he may not learn or cast any spells associated with the element that opposes his element of specialty, and is also prohibited from using magical items that duplicate spell effects of his oppositional element.

Note: Elementalism originally appeared in this form in the Tome of Magic for the AD&D game from TSR. Its use here is for the purposes of providing context for the campaign only.