*If and only if* is an important logical concept, useful in many contexts, both mathematical and nonmathematical. Unfortunately, "if and only if" is also an unwieldy five-syllable phrase. Mathematicians have solved this problem by shortening it to "iff". Unfortunately, this shortening has not caught on in non-mathematical contexts. This makes some communication and thinking unwieldy and ambiguous.

I think the reason "iff" hasn't caught on more broadly is because it's easily misread as "if", and doesn't have an intuitive pronunciation. I think both of these problems would be solved by changing the spelling to "ifeff" (prononunced /ɪfɛff/). The etymology is that you take "iff", and pronounce the second "f" separately. This would slightly improve the thinking and communication of most English speakers.

I think a small group of people using "ifeff" in their writing would likely start a process where "ifeff" eventually takes over, via the usual process by which vocabulary spreads, and that "ifeff" would be used by groups that don't currently have a short-enough word for this concept. I also think the correspondence between "iff" and "ifeff" is intuitive enough that this will not cause very much confusion.