## Iff

If and only if (i.e., Necessary and Sufficient). The terms ``Just If'' or ``Exactly When'' are sometimes used instead. iff is written symbolically as . iff is also equivalent to , together with , where the symbol denotes ``Implies.''

J. H. Conway believes that the word originated with P. Halmos and was transmitted through Kelley (1975). Halmos has stated, ``To the best of my knowledge, I DID invent the silly thing, but I wouldn't swear to it in a court of law. So there--give me credit for it anyway'' (Asimov 1997).