Ephemeralism, also referred to as Astronic mortalism, or sublunism, is a post-omnidoxical thanatological belief within the Astronist tradition, or Astronism's primary view on the topic of death. It upholds the notion that all that does exist and holds its inception within The Cosmos is transient, or naturally impermanent due to the effects of Cosmic Limitation.

Therefore, it is our nature as cosmic beings to die, or to be mortal, or to be of a transient existence. The ephemeralist view rejects the notion of immortality due to the fact that immortality pertains to a type of temporal and existential infiniteness which defies the principle of our limited natures. Hence, in Astronism, death is considered a natural part of life and is not something to be feared, but simply accepted for what it is; the conclusion to our individual transient existence. For this reason, there is no sense of mourning with regards to death in Astronism, but instead, a feeling of rejoice for those whom have transformed into The Cosmos.

In the ephemeralist tradition, those whom have passed away are considered "to become One with The Cosmos" or "to transform into The Cosmos", as it is believed that upon our deaths, our entire selves become The Cosmos and cease to exist as composite entities.

Ephemeralism is a belief that is heavily influenced by the naturalist school of thought of Millettarian eschatology and can be considered an extension to the beliefs of naturalism.