ne·ces·si·ty 
(nə-sĕsĭ-tē)
(nə-sĕsĭ-tē)n. pl. ne·ces·si·ties
Idiom: 1.
a. The condition or quality of being necessary.
b. Something necessary: The necessities of life include food, clothing, and shelter.
2.
a. Something dictated by invariable physical laws.
b. The force exerted by circumstance.
3. The state or fact of being in need.
4. Pressing or urgent need, especially that arising from poverty.
of necessity
As an inevitable consequence; necessarily.
[Middle English necessite, from Old French, from Latin necessitās, from necesse, necessary; see NECESSARY.]