indispensable

/ˌɪndɪˈspɛnsəbəl/
adjective
  1. Absolutely necessary or essential; cannot be done without.
    • A good dictionary is indispensable for learning a new language.
    • Her calm leadership was indispensable during the crisis.
    • Water is indispensable for all forms of life.
noun
  1. A person or thing that is absolutely necessary or essential.
    • In the kitchen, a sharp chef's knife is an indispensable.
    • She quickly became an indispensable to the team because of her problem-solving skills.
    • For a long hike, a map and compass are indispensables.