essential

/ɪˈsɛnʃəl/
adjective
  1. Absolutely necessary; extremely important.
    • Good communication is essential in a healthy relationship.
    • It is essential that you bring your passport to the airport.
    • Water is essential for all living things.
  2. Relating to the basic or most important part of something; fundamental.
    • The essential difference between the two plans is the cost.
    • We need to focus on the essential elements of the problem.
    • She explained the essential points of the theory.
noun
  1. A thing that is absolutely necessary or very important.
    • The store sells basic essentials like bread and milk.
    • Patience is an essential for any good teacher.
    • A warm coat is an essential for a winter trip to Canada.