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.
永遠の命 means 'eternal life' and is more common in everyday speech and religious contexts. 不死性 is more technical or philosophical.
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.