Synonyms of the word

indispensable

• adj. (ecclesiastical, obsolete) Not admitting ecclesiastical dispensation; not subject to release or exemption;…
• adj. (of duties, rules etc.) Unbendable, that cannot be set aside or ignored.
• adj. Absolutely necessary or requisite; that one cannot do without.
• n. A thing that is not dispensable; a necessity.
• n. (in the plural, colloquial, dated) Trousers.

critical

• adj. Inclined to find fault or criticize; fastidious; captious; censorious; exacting.
• adj. Pertaining to, or indicating, a crisis or turning point.
• adj. Extremely important.
• adj. Relating to criticism or careful analysis, such as literary or film criticism.
• adj. (medicine) Of a patient condition involving unstable vital signs and a prognosis that predicts the condition…
• adj. Likely to go out of control if disturbed, that is, opposite of stable.
• adj. Of the point (in temperature, reagent concentration etc.) where a nuclear or chemical reaction becomes…
• n. A critical value, factor, etc.
• n. In breakdancing, a kind of airflare move in which the dancer hops from one hand to the other.

essential

• adj. Necessary.
• adj. Very important; of high importance.
• adj. (biology) necessary for survival but not synthesized by the organism, thus needing to be ingested.
• adj. Being in the basic form; showing its essence.
• adj. Really existing; existent.
• adj. (of a lamination of a 3-manifold) Such that each complementary region is irreducible, the boundary of…
• adj. (medicine) Idiopathic.
• n. A necessary ingredient.
• n. A fundamental ingredient.

necessary

• adj. Required, essential, whether logically inescapable or needed in order to achieve a desired result or avoid…
• adj. Unavoidable, inevitable.
• adj. (obsolete) Determined, involuntary: acting from compulsion rather than free will.
• n. (Britain, archaic euphemistic, usually with the definite article) A place to do the "necessary" business…

obligatory

• adj. Imposing obligation, morally or legally; binding.
• adj. Requiring a matter or obligation.

vital

• adj. Relating to, or characteristic of life.
• adj. Necessary to the continuation of life; being the seat of life; being that on which life depends.
• adj. Invigorating or life-giving.
• adj. Necessary to continued existence.
• adj. Relating to the recording of life events.
• adj. Very important.
• adj. Containing life; living.
• adj. Capable of living; in a state to live; viable.