Essential requirement
Essential requirement, indispensable thing/item, requisite, fundamental, sine qua non, indispensability, needfulness, prerequisite, essential, necessary, basic, desideratum, and need.
Inevitability
Inevitability, certainty, inexorability, unavoidability, inescapability, and ineluctability.
Need
Force/pressure of circumstance, obligation, exigency, emergency, force majeure, need, call, crisis, and urgency.