Ethereal Waves are the very thing that enables magic.
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.
|“|| The second component required for spellcasting is the proper reagent. These chemical materials serve as a link between the physical world that the mage wishes to affect and the psychic energies of the ethereal waves that the mage is drawing upon to effect that change. Some reagents are relatively common herbs, others are rare and exotic items.|
There are eight circles of magical spells, each successive circle representing a more powerful level of magical energy. The ethereal waves are made up of eight rings or circles which correspond to the levels of the spells. Each circle is one of eight swirling vortices, all spinning one within another and expanding out through the universe. The more powerful the spell a mage casts, the deeper into the inner circles of the ether must the concentrated psychic energy of the mage penetrate.