essential; indispensable
inevitable; inescapable
seeing that death, a necessary end, will come when it will come — Shakespeare
logically unavoidable
a necessary conclusion
in philosophy, said of a statement that cannot be denied without contradiction of some other statement, e.g. ‘All spinsters are unmarried’
acting under compulsion; not free
a necessary agent
[Middle English from Latin necessarius, from necesse needful, unavoidable]