The Oxford Thesaurus An A-Z Dictionary of Synonyms - necessity n.

1) requirement, essential, necessary, requisite, need, prerequisite, basic, fundamental, sine qua non, desideratum, constraint: I was marooned for a year with only the bare necessities of life. We regard honesty as a necessity in a bank manager. 2) indispensability, unavoidability, needfulness, inexorability: The necessity for exercise is often met by private health clubs. 3) poverty, want, indigence, need, destitution, penury, straits, difficulty, difficulties, pauperism, neediness: The extreme necessity of the unemployed demands that the government act quickly. 4) urgency, emergency, crisis, misfortune, exigency, pinch, extreme, matter of life and death: She has made a virtue of necessity. Electric power is no longer a matter of choice but of necessity.