Indispensable, a.
Etymology: [Pref. in- not + dispensable: cf. F. indispensable.]
1. Not dispensable; impossible to be omitted, remitted, or spared; absolutely necessary or requisite.
2. (Eccl.)
Not admitting dispensation; not subject to release or exemption. [R.] The law was moral and indispensable. Bp. Burnet.
3. Unavoidable; inevitable. [Obs.] Fuller.
Shades of meaning
Fernald has no entry of its own for indispensable; he lists it under inherent, necessary.
Where he writes about it:
That which is essential belongs to the essence of a thing, so that the thing can not exist in its completeness without it; that which is indispensable may be only an adjunct, but it is one that can not be spared; vigorous health is essential to an arctic explorer; warm clothing is indispensable. — from necessary
That which is requisite (or required) is so in the judgment of the person requiring it, but may not be so absolutely; thus, the requisite is more a matter of personal feeling than the indispensable. — from necessary