Use this tag to mark questions related to creating plausible alien biologies that may be close to or far removed from Earthly biology but within the realms of hard science.
See Wikipedia entry. Xenobiology describes a form of biology that is not familiar to science and is not found in Earthly life. Novel biological systems and biochemistries that differ from the canonical DNA-RNA-20 amino acid system, which can include variations on the same these (e.g. different amino acids), totally different systems (such as the SF staple of silicon-based life).
This can include different biochemistry, or different biological systems and organs without regard to the underlying chemestry.