Before we discuss actual examples of unearthly life, it would be prudent to set forth a solid definition of what life actually entails. For my own worldbuilding purposes, I have composed a simple set of requirements for something to be defined as alive:
> Inertia-independent reaction to stimulus, however slight and regardless of speed.
> The independent production/collection and storage of energy within the entity in question for biological use.
> The ability of the entity in question to replicate all or parts of itself, independently, regardless of the type of biophysical unit replicable; or the ability to reproduce sexually; or the ability to replicate its consciousness.

(Personally, I am of the opinion that if a biologically dubious entity can be conclusively proven to be sapient, it should be defined as technically alive, regardless of the existential status of any of the above requirements. However, this is a rather subjective attribute and can be discarded for more rigorous classification.)
With this minimal yet solid definition of life on hand to identify living things of all kinds, we may now venture into the speculative cosmic wilderness.

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