noun
Refers to the state or quality of being immortal, often used in philosophical, mythological, or biological contexts.
不死性を求めることは、多くの神話のテーマだ。
The pursuit of immortality is a theme in many myths.
ある種のクラゲは生物学的な不死性を持つと言われている。
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.