Required, essential, whether logically inescapable or needed in order to achieve a desired result or avoid some penalty.
Unavoidable, inevitable.
Necessary to continued existence.
Invigorating or life-giving.
Very important.
Containing life; living.
Necessary to the continuation of life; being the seat of life; being that on which life depends.
Capable of living; in a state to live; viable.
Relating to the recording of life events.
Relating to, or characteristic of life.