► necessaryneeded in order for you to do something or have something: You’ll find all the necessary information in this booklet. The bad weather made it necessary for us to change our plans.
► essential necessary in order for something to happen or be done: Oxygen is essential for animals and plants to live.
► vital something that is vital is extremely important and necessary so that without it there will be serious problems: He was accused of withholding vital information from the police.
► crucial/critical something that is crucial or critical is important and necessary because other things depend on it: The information was crucial to building a case against the accused murderer. Having a positive attitude is critical to achieving your goals.
► mandatory/compulsory/obligatory something that is mandatory, etc., must be done because of a rule or law: Parents do not want school uniform to become mandatory. Service in the army was compulsory.
► requisite formal needed for a particular purpose: He lacked the requisite skills for the job.
► indispensable formal something or someone that is indispensable is so important or useful that something cannot be done without that person or thing: The accounting program is indispensable for small businesses.
► imperative extremely important and needing to be done: It is imperative that you report back to me as soon as you are finished.