Spirits are any of a variety of demi-humans that are either incorporeal manifestations of the ethereal realm or a revenant species such as a ghost. The term is primarily used informally and has no formal definition, as various demi-humans may prefer to either wear the term or not. For instance, elementals usually prefer to use the term while deities do not. As a rule of thumb, demi-humans which have a physical form based on biological matter, whether or not it is augmented by magic, are generally not considered spirits, while demi-humans whose physical form is a mirage or who do not have a physical form, including demi-humans who possess living beings to live on earth, are considered spirits.
As such, the term "spirit" exists primarily for the sake of personal identity and is a social concept rather than a rigorous one. It is a core concept in demi-human pride in cities such as Offetstine, where spirits are especially common, and where the term "spirit" is commonly used to symbolize a detachment from human society, or the property of being born of the human id and the Will of the People. Spirits often interact directly with humans and live among them, but often do not occupy typical societal roles, instead presiding over matters of the human psyche. Many spirits live by a guiding principle, or are more in tune with their ethereal nature than their human nature. For many spirits, their human nature is alienable to them, and merely a construct they embody. Many spirits similarly do not have any human traits at all. For instance, while forest spirits such as nymphs usually have humanlike traits, sylphs are often purely incorporeal air spirits with no human traits.
Spirits are usually classified either Aetherius or Caelestis under the Émile standard for demi-human species taxonomy, though some species, especially revenants are considered Infernalis instead. Spirits that do not have human traits often remain unclassified under the Émile standard, as it is often hard to work with them to classify them, and the Émile standard was built on the principle of helping affirm personal identity. Usually spirits without human traits are given temporary taxa that are not codified for a very long time.