From House of Hozz

The info here is in a state of flux, everything can be changed!

Aether is the substance in which all other substances exist. It has density, just like all other substances, but is not affected by the presence of any other substance. High densities of Aether result in solid-clouds, which form the boundary between TwinWorld and the Outer Fire.

Aether has the effect of dampening motion, and is the universal reference frame in Twinworld. Among other things, this means that solid-clouds delay the transmission of light, and time moves more slowly inside of them? Whaaaat? Need to think about the implications of this a bit more.

Aether also affects enchantment, generally increasing the strength, range, and duration with increased Aether density. For this reason, the best enchantments are made in the upper branches of the Great Tree, or in outposts in the solid-clouds themselves. However, since solid-clouds can not be moved or changed, the effect on the enchantment only endures to the extent that the enchantment remains in the vacinity of high aether concentrations.

Small concentrations of aether are known as "knots" or "whorls". The Heart of Water is one such whorl.