Controlling force; irresistible compulsion; a power or impulse so great that it admits no choice of conduct. That which makes the contrary of a thing impossible. The quality or state of being necessary, in its primary sense signifying that which makes an act or event unavoidable. Quality or state of fact of being in difficulties or in need; a condition arising out of circumstances that compels a certain course of action. Bykofsky v. Borough of Middletown, D.C.Pa., 401 F.Supp. 1242, 1250.
A person is excused from criminal liability if he acts under a duress of circumstances to protect life or limb or health in a reasonable manner and with no other acceptable choice.
The word "necessity", within certificate of public convenience and necessity, is not used in the sense of being essential or absolutely indispensable but merely that certificate is reasonably necessary for public good. Alabama Public Service Commission v. Crow, 247 Ala. 120, 22 So.2d 721, 724.
To fulfill requirements for easement of right of way of necessity, the necessity must be actual, real, and reasonable, as distinguished from inconvenience, but it need not be absolute and irresistible necessity. When used in relation to power of eminent domain does not mean absolute necessity, but only reasonable necessity
The "necessity" of and appurtenance for the beneficial use of leased premises, which will entitle the lessee thereto, is not an absolute necessity in the sense that it must be completely indispensable, but is a real necessity and not a mere convenience or advantage.
See also necessaries

Black's law dictionary. . 1990.