An Etymological Dictionary of Astronomy and Astrophysics

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.

Etymology (EN): M.E. necessaire, from L. necessarius “unavoidable,” , from necesse “unavoidable, indispensable,” from ne- “not,” → un-, + cedere “to withdraw, go away, yield,” → precession.

Etymology (PE): 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.