WordNet sense 1 (anything indispensable):

necessity, requirement

WordNet sense 2 (required by obligation or compulsion or convention):

obligatory

WordNet sense 3 (necessary for relief or supply):

needful, requisite, needed, required

WordNet sense 4 (absolutely necessary; vitally necessary):

essential, indispensable

WordNet sense 5 (incapable of being avoided or prevented):

inevitable