indispensable in WordNet English dictionary

1. absolutely necessary; vitally necessary
   "essential tools and materials"; "funds essential to the completion of the project"; "an indispensable worker"
   essential  indispensable 
   
2. not to be dispensed with; essential
   "foods indispensable to good nutrition"
   indispensable 
   
3. unavoidable
   "the routine but indispensable ceremonies of state"
   indispensable