- If and only if. This is a mathematical abbreviation.
A iff B indicates that A will only ever be true if condition B is true and not on any other condition. Thus "A iff B" is not the same as "A if B" because "A if B" allows for the possibility of some other condition such as "A if C".
For more information see:
- http://en.wikipedia.org/wiki/Iff - Extensive definition of Iff, containing distinctions between if and iff.