These terms are used almost exclusively among intangible beings, though they also circulate among those with extensive dealings with such disembodied creatures.

Concreta

Creatures with physical bodies are called concreta. Whether planar or mortal, they have material bodies that define them. They exist exclusively and irrevocably in physical reality. The astral realm is the most rarefied plane their bodies can attain.

Abstracta

Abstracta are the spirits of ideas, actual thought given form in a realm beyond the most obscure reaches of the astral plane. They see the world as an ecology of ideas. While their actions can have profound impact on the world of more concrete beings, most remain as oblivious to our world as we are to theirs.

Abstracta are not incorporeal – they have no discernible form at all. They only exist in the abstract realms, from which they project their influence into the world at large. They are riddles, dreams, rumors, and archetypes. They are everywhere yet nowhere, present yet intangible.

Though known by a variety of names, abstracta are all manifestations of a particular concept. Much like outsiders, they gain essence by playing certain roles or producing certain outcomes. Like outsiders, their existence is tenuous. While they do not age or die of natural causes, others of their kind can destroy them for their essence, and they can starve to death. Like outsiders, abstracta can experience crux events, possibly resulting in level loss.

Abstracta can suffer another fate: reification. If an abstractum becomes too involved with a particular manifestation of its concept, its obsession can cause it to fall from its abstract state and become ephemeral. This is a one-way trip – once abstracta catch a glimpse of concrete reality, they tumble from the abstract realms forever.

Ephemera

Ephemera occupy concrete reality, but they lack concrete bodies. They are incorporeal in all planes, and can reduce their apparent size by one size category to fit “within” small objects or spaces. Many ephemera are invisible by default, or may become invisible at will.

Ephemera are creatures of essence, but they are alien even to other outsiders. Their strong ties to the abstract realms lead them to interact with the world in ways that others cannot entirely understand, much less emulate.

While some ephemera can reproduce, the vast majority of them are refugees from the abstract realms. Due to their abstract heritage, they can briefly project into the abstract realms (for one round per HD, at will).

Not all incorporeal beings are ephemera. Ephemera are incorporeal in all frames of reference, not physically lurking on one plane and projecting into another. You cannot travel to another plane to do physical battle with such creatures.

Though not cut from the same cloth, shadows and similar incorporeal beings could also be considered ephemera. They are incorporeal in all frames of reference, although their origin is decidedly non-abstract. It is a matter of conjecture as to whether such beings can project into the abstract realms. The thought of hordes of shadows casting their darkness upon thoughts, dreams, and ideas is ipso facto the stuff of nightmares.