1) Absolutely necessary 2) Critical 3) French word used in English 4) Necessary 5) Not to be dispensed with 6) Unavoidable 7) Vital 8) Vitally necessary 1) Central 2) Critical 3) Essential 4) Key 5) Necessary 6) Needed 7) Obligatory 8) Vital 
[1791] The Indispensable, also known as Indispensible, was a 351 ton sailing ship built in France. She was taken up for service with the British East India Company between 1793-1797. She sailed to Port Jackson in May 1794 departing for Bengal in July 1794. On her first convict voyage under the command of William Wilkinson, she sailed from E...
• (a.) Not dispensable; impossible to be omitted, remitted, or spared; absolutely necessary or requisite. • (a.) Unavoidable; inevitable. • (a.) Not admitting dispensation; not subject to release or exemption.
In`dis·pen'sa·ble adjective [ Prefix in- not + dispensable : confer French indispensable .] 1. Not dispensable; impossible to be omitted, remitted, or spared; absolutely necessary or requisite. 2. (Eccl.) Not admitting dispensation; not subject to release...
[adj] - not to be dispensed with 2. [adj] - unavoidable
adjective unavoidable; `the routine but indispensable ceremonies of state`
absolutely necessary, essential, or requisite: an indispensable member of the staff. · incapable of being disregarded or neglected: an indispensable obligation.