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general::
compulsory
if something is compulsory, you must do it because of a rule or law:
• Maths and English are compulsory for all students. • Compulsory education was introduced in 1870. • Wearing a seatbelt is compulsory. • It is now compulsory for anyone claiming state benefit to register with a job centre.
obligatory
[ not usually before noun ] if something is obligatory, you must do it because of a rule or law. Obligatory is more formal than compulsory:
• It is now obligatory for all competitors to wear face protectors. • Evening dress is usual, but not obligatory. • Military service is obligatory for all men between 18 and 27.
mandatory
if something is mandatory, you must do it because it is the law. Mandatory is more formal than compulsory and sounds stronger:
• Drug smuggling carries a mandatory death sentence. • In some countries, wearing helmets is mandatory for all cyclists.
not compulsory
optional
if something is optional, you can choose whether to do it or not:
• You don’t have to do French – it’s optional. • The holiday price includes optional excursions such as an evening cruise on the lake. • General Studies is optional for sixth-form students.
voluntary
done because you want to do it and not because you have to – used especially about things that you do because you think they will help other people or be useful:
• The programme is entirely voluntary, and no one will be forced to join. • The charity is funded by voluntary donations.
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