The stuff of imagination.
Imaethyr is the material of which everything imagined is made of. It is the fundamental element of imagination. Every time anyone imagines something, that imagined something is made of Imaethyr.It exists in the Imaginarium and fills it entirely. Everything in the Imaginarium is made of it to some extent. The Deep Imaginarium is also probably filled with and made out of it, possibly molded into something slightly different in many places. Most likely that something else is just a different state of the same stuff, or possibly the same Imaethyr in a slightly different composition.
It is very different from what is normally understood as a material. It is somewhat fluid like in its nature. A thick, high viscous fluid. It appears to form very chaotic blobs mostly composed by strange whirlwind like structures. It is fractal in nature so the structure at any one point is always equal to the overall structure. It can be infinitely divisible into smaller and smaller parts and it looks the same at any zoom level. The smaller parts are just like the larger ones and it is not possible to compare or differentiate their sizes either. For these reasons even the term "quantity" is not well defined when applied to it. It probably does not have the property of quantity at all.
It is impossible to say if the process of imagining something actually creates an additional amount of Imaethyr or if it only creates the things imagined and they are made of already existent Imaethyr.
It does not have the property of density at all. It is not that it has 0 density, it doesn't have the property. Given any amount of space, any size of a container, it is always possible to put an infinite amount of it inside that space without any changes to its properties or structure and without the space being affected by it in any way.
It is permeable to everything and it can function somewhat like space in that sense. Anything can penetrate and move through it easily without affecting or being affected by it in any way. It is non-interacting with other things in that sense. It does not interact with or affect things inside it in any way. It imposes no limits like limiting how fast things can move inside it, it has no mass or drag coefficient, no attrition, etc.
It is malleable to the extreme and can be shaped and molded into becoming anything else and having any properties desired. It can be given density, either 0 density or a positive or negative density of any value. Matter and energy can be made out of it. Even space-time fabric can be made out of it. One can even make nothing out of it.
The willpower and the act of imagination molds it easily and naturally. Any limitations on what imagination can do to it is not a limitation of its malleability but, instead, a limitation of the mind doing the imagining.
Essentially it is the fundamental essence. With certainty it is the fundamental essence of everything that is not strictly real. If there is a fundamental essence of reality as well it can also be it but that is not a certainty, so it is not known if the Realium is also made out of it or not. The only thing that is known is that everything in the Realium can be made out of it but not if it actually is or not.