obligatory

Từ điển WordNet v3.1 - WordNet Dictionary

adjective

1. morally or legally constraining or binding
- attendance is obligatory
- an obligatory contribution
• Ant:
optional
• Similar to:
bounden, compulsory, mandatory, required, de rigueur,
imposed, incumbent on, indispensable, prerequisite
• See Also:
necessary
2. required by obligation or compulsion or convention
- he made all the obligatory apologies
• Similar to:
necessary