Compulsory means required by the rules — you have no choice about it. Compulsory education, a compulsory meeting, wearing a helmet is compulsory.
It is a British word for the most part; American English usually says mandatory or required. Compulsory describes the requirement; obligatory describes the duty, and the two are close enough that either usually works. The opposite is optional or voluntary. Note the noun compulsion, which has drifted somewhere else entirely: a compulsion is an inner urge you cannot resist, not an outer rule, which is why compulsive and compulsory are not the same word at all.
1adjectivecompulsoryRequired by rule or law.
“Attendance at the briefing is compulsory.”
synonyms: mandatory, obligatory, required
2adjectiveschoolOf a school subject, one that must be taken.
“Maths is compulsory until sixteen.”
compulsory is a state of being required.
Also accepted in games: mandatory, obligatory
Latin compellere meant to drive together, then to force — com-, together, plus pellere, to drive, the same verb behind propel, expel and pulse. Something compulsory drives you, and the word keeps the pressure in it.
Latin
compulsorius
English, today
compulsory
Latin compulsorius, from Latin compulsus. Displaced native ġenīedelīċ.
Etymology from Wiktionary, CC BY-SA
Practised in Sharper Adjectives, School Subjects