ne·ces·si·ty noun 1. the fact of being required or indispensable. "the necessity of providing parental guidance should be apparent" Similar: essential requirement prerequisite indispensable thing/item essential requisite necessary fundamental basic sine qua non desideratum indispensability need needfulness 2. an indispensable thing. "a good book is a necessity when traveling" Similar: essential requirement prerequisite indispensable thing/item