Definitions
from The American Heritage® Dictionary of the English Language, 5th Edition.
 abbreviation if and only if
from Wiktionary, Creative Commons Attribution/ShareAlike License.
 abbreviation mathematics, logic
if and only if ; used to show that thepredicate that follows it has the sametruth value as the predicate that precedes it.
Etymologies
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Examples

This may get a little confusing but the difference for the sake of argument between 'if' and 'iff' is as follows:

˜Snow is white™ is true in English iff snow is white.

By the way, the use of 'iff' in the introduction is not a typo, it's a shorthand used by philosophers for if and only if.

By the way, the use of 'iff' in the introduction is not a typo, it's a shorthand used by philosophers for if and only if.

For readers who might not know, in logic 'iff' means 'if and only if', something I explained to the class several times, and which was ACTUALLY MENTIONED on the handout.

But did you know that if you don't need an 'else', you can simply type "iff", press Tab, and then end up with this: if (exp) {

EagleeyeNASA, iff (I used iff deliberately) the Aerospace Corp.
Jeff Hanley's Latest Update From the Denial Zone  NASA Watch

I know that Plaid has little opposition in Gwynedd but good grief, this man is iff his trolley?

"Yes, the iff card sounds peculiar, but then the oyster card was pretty weird at first too eh?"

We are going to have accept there is going to be some risk, but maybe its worth living with ambiguity in the hope of somehow heading them off from exercising that choice, because iff you strike the Iranian programme you guarantee they are going to turn around and try to make a bomb.
Iran nuclear report: IAEA claims Tehran working on advanced warhead
whichbe commented on the word iff
"If and only if."
May 7, 2008
pterodactyl commented on the word iff
I like this word. It makes me feel nostalgic, because I saw a lot of it back in college when I was getting my philosophy degree.
May 8, 2008
vanishedone commented on the word iff
Trouble is, trying to pronounce it without making it confusible with if can be a bit iffy.
Then again, it means one can write things like 'If "if P then (Q iff R)" is a wff iff "if (if P then Q) iff R" is a wff, then iff "iff P then (Q iff R)" is a wff, if "P iff (Q iff R)" is a wff, "(P iff Q) iff R" is a wff'.
Just in case you wanted to do that sort of thing.
May 8, 2008
pterodactyl commented on the word iff
I do! I do want to do that sort of thing!
And unless I'm mistaken, your statement works out as "true". :)
May 9, 2008
hernesheir commented on the word iff
I will iif you will.
October 1, 2011
yarb commented on the word iff
Interesting to see widespread use of this bit of philosophical jargon on the Twitter.
Who said Twitter was an intellectual desert?
October 1, 2011