Synonyms: compulsory, mandatory, required
Definition: required by rule
Usage: in most schools physical education is compulsory; attendance is mandatory; required reading
Similar words: obligatory
Definition: morally or legally constraining or binding
Usage: attendance is obligatory; an obligatory contribution
Synonyms: needed, needful, required, requisite
Definition: necessary for relief or supply
Usage: provided them with all things needful
Similar words: necessary
Definition: absolutely essential