indispensable (
http://definr.com/indispensable)
adj 1: not to be dispensed with; essential; "foods indispensable to
good nutrition" [ant:
dispensable]
2: absolutely necessary; vitally necessary; "essential tools
and materials"; "funds essential to the completion of the
project"; "an indispensable worker" [syn:
essential]
3: unavoidable; "the routine but indispensable ceremonies of
state"