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Mathematics.

if and only if.

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Question 1 of 7

Fill in the blank: I can’t figure out _____ gave me this gift.

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Words nearby iff

If at first you don't succeed, try, try again, IFC, if-come bet, Ife, -iferous, iff, iffen, iffy, if it ain't broke don't fix it, if looks could kill, I.F.L.W.U.

Other definitions for iff (2 of 2)

IFF

Military.

Identification, Friend or Foe: a system of transmitters and transponders that uses coded signals to distinguish between friendly and hostile aircraft.

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

How to use iff in a sentence

The IFF, however, is not exactly an organization to be proud of.
The Real Jersey Dictionary, Vol. 2|Jaimie Etkin|October 21, 2010|DAILY BEAST
It was a kind of stick called vahs-iff, which did not grow there, therefore they had not been able to find it.
Aw-Aw-Tam Indian Nights|J. William Lloyd
It would more encurrege to wed, iff so be one know'd it before-hand, as one may say.
Clarissa, Volume 3 (of 9)|Samuel Richardson
The IFF mechanism reconverts our beam to a predetermined pulse, and it bounces back to us in a different pattern.
Progress Report|Mark Clifton

And what wold ye doe with my bryde soe gay, Iff I did sell her thee?
A Book of Ballads, Volume 1|Various
Now while Staff was receiving this impression, Mr. Iff looked sharply round; their glances crossed.
The Bandbox|Louis Joseph Vance

British Dictionary definitions for iff (1 of 2)

iff

/ (ɪf) /

conjunction

logic a shortened form of if and only if : it indicates that the two sentences so connected are necessary and sufficient conditions for one another. Usually iff is used for equivalence in the metalanguage, rather than as the biconditional in the object language

British Dictionary definitions for iff (2 of 2)

IFF

/ military /

abbreviation for

Identification, Friend or Foe: a system using radar transmissions to which equipment carried by friendly forces automatically responds with a precoded signal

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012