Iff
If and only if (i.e., Necessary and Sufficient). The terms ``Just If'' or ``Exactly When'' aresometimes used instead. 
iff 
is written symbolically as 
. iff is alsoequivalent 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--giveme credit for it anyway'' (Asimov 1997).
See also Equivalent, Exactly One, Implies, Necessary, Sufficient
References
Asimov, D. ``Iff.'' math-fun@cs.arizona.edu posting, Sept. 19, 1997. Kelley, J. L. General Topology. New York: Springer-Verlag, 1975.
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