necessary (needed to have or do in order to make a situation possible): No experience is necessary for this job. Oxygen is necessary for life on earth.
essential (extremely necessary and important): It is essential to prepare for the exam.
vital (essential for something to succeed or exist): Vitamins and minerals are vital for good health.
compulsory formal (required to be done because of a rule or law): Wearing helmets is compulsory for all cyclists.
mandatory formal (required to be done because of a mandate or law): Attendance to the meeting is mandatory.
obligatory formal (required to be done in order to obey a law or rule): Wearing protective clothing is obligatory for all employees.
requisite formal (essential for a particular purpose): Mike has the requisite skills for this job.
imperative formal (vitally important): It is imperative that we inform the public about these dangers.
