Indispensable
Part of speech: noun
Definition: A thing that is not dispensable; a necessity. Trousers.
Part of speech: adjective
Definition: Not admitting ecclesiastical dispensation; not subject to release or exemption; that cannot be allowed by bending the canonical rules. Of rules, duties etc: unbendable, that cannot be set aside or ignored. Absolutely necessary or requisite; that one cannot do without.
Example sentence: Two qualities are indispensable: first, an intellect that, even in the darkest hour, retains some glimmerings of the inner light which leads to truth; and second, the courage to follow this faint light wherever it may lead.
Vital
Part of speech: adjective
Definition: Relating to, or characteristic of life.Necessary to the continuation of life; being the seat of life; being that on which life depends.Invigorating or life-giving.Necessary to continued existence.Relating to the recording of life events.Important
Example sentence: If I see anything vital around me, it is precisely that spirit of adventure, which seems indestructible and is akin to curiosity.