Main entry: obligatory
Definition: morally or legally constraining or binding
Usage: attendance is obligatory; an obligatory contribution
Antonyms: optional
Definition: possible but not necessary; left to personal choice
Antonyms: dispensable
Definition: capable of being dispensed with or done without
Main entry: obligatory
Definition: required by obligation or compulsion or convention
Usage: he made all the obligatory apologies
Antonyms: unnecessary, unneeded
Definition: not necessary