► necessary used to describe something that you need to have or do: · Make sure you bring the necessary documents with you.· It may be necessary for you to have a small operation.
► essential very important and necessary, especially in order to be healthy, successful etc: · Vitamins are essential for healthy growth.· The tourist industry is an essential part of the Spanish economy.
► vital extremely important and necessary, especially in order to avoid serious problems: · A vital piece of equipment on the spacecraft had stopped operating.· It is vital that the aid is sent immediately.
► compulsory if something is compulsory, you must do it because of a rule or law: · Maths and Science are compulsory subjects.· All new staff undergo a compulsory training course.
► obligatory if something is obligatory, you must do it because of a rule or law. Obligatory is more formal than compulsory: · The use of seatbelts is obligatory.· Safety regulations have made it obligatory for all competitors to wear fist protectors.
► mandatory if something is mandatory, you must do it because it is the law. Mandatory is more formal than compulsory and sounds stronger: · School attendance is mandatory.· a prisoner serving a mandatory life sentence
► requisite formal [usually before noun] the requisite things are the ones that you need to have in order to do something: · The other candidates lacked the requisite skills.· the requisite evidence needed for a successful prosecution