adjective
1.
Synonyms: needful , needed, requisite
2.
Synonyms: mandatory , compulsatory, compulsory, imperative, imperious, obligatory
Antonyms: optional
adjective
1.
Synonyms: needful , needed, requisite
2.
Synonyms: mandatory , compulsatory, compulsory, imperative, imperious, obligatory
Antonyms: optional
Merriam Webster. Collegiate thesaurus English vocabulary. Английский энциклопедический словарь тезаурус. 2003