Ethereal Waves

Ethereal Waves are the very thing that enables magic.

Description
The exact nature of ethereal waves was never much discussed during the series. From what can be gathered, it is these very waves that enable the usage of magic in the first place, acting as the power source and carrier of the magical arts. These waves originate in the Ethereal Void, where they are created by circles of magical energy. The deeper the circle, the more powerful the waves that originate from them. The arts of magic, involving reagents and words of power, try to harness the waves into the wanted result.

The waves do interact with their surroundings. The material blackrock can block off the waves, proving that the waves have some of the properties of radiation. Enough quantities of blackrock make magic completely unusable, such as the Blackrock Dome and the dungeon Covetous in Ultima IX.

In Ultima VII, the Tetrahedron Generator twisted the waves in the ether surrounding Britannia, turning them sharp instead of smooth, thus making magic unreliable. It also demonstrates that mages are so attuned to the waves, that these disruptions also disrupt their minds.

Trivia

 * The number of circles from which the waves originate seems different for every plane. Britannia has the Eight Circles of Magic, while the Serpent Isle has nine.