Refers to the state or quality of being immortal, often used in philosophical, mythological, or biological contexts.
The pursuit of immortality is a theme in many myths.
Some jellyfish are said to possess biological immortality.
Compound of 不死 (fushi, 'immortal') + 性 (sei, '-ity, -ness'), forming an abstract noun meaning 'immortality'. The exact historical derivation is straightforward from these components.