absolutely necessary, essential, or requisite

an indispensable member of the staff.

incapable of being disregarded or neglected

an indispensable obligation.

a person or thing that is indispensable.

adjective

dispensable , needless , nonessential , redundant , superfluous , unnecessary

adjective

basal , basic , cardinal , crucial , essential , fundamental , imperative , key , necessitous , needed , needful , prerequisite , primary , required , requisite , vital , necessary , de rigueur , integral , obligatory , sine qua non , unavoidable