An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

1) Being essential or indispensable.
2) Logic, Math.: A condition which must hold for a result to be true, but which does not guarantee it to be true. → if and only if.

M.E. necessaire, from L. necessarius "unavoidable," , from necesse "unavoidable, indispensable," from ne- "not," → un-, + cedere "to withdraw, go away, yield," → precession.

Bâyesté, p.p. of bây-, bâyestan "to be necessary," from Mid.Pers. abây-, abâyistan "to be necessary" (abâyišn "necessity," abâyišnig "necessary"), from Proto-Ir. *upa-aya- "to reach," from upa-, → hypo-, + ay- "to go, to come," → precession.

If event A must occur for event B to occur, then it is said that A is → necessary for B. If event A may cause B but there could be some other cause as well, then it is said that A is sufficient to cause B. See also → if and only if (iff).

→ necessary; → and; → sufficient; → condition.

Logic: A → proposition if its → denial is self-contradictory. Also called "logical truth" and "non-contingent truth."

→ necessary; → truth.