I think a better approach is to keep using `iff' (also known as `ssi' in
French), but to make it such that the user can click on it to get to the
Glossary where we add an entry for it.
We could later on use the same feature for some other terms (e.g.
"fringe").
The problem with using "iff" is that some readers will not notice that it is
not "if", or they might think that it is a typo. If we use "iff" as Stefan
suggested, then it is important that users have a _visual_ clue that this is
a glossary term and that they can click it to see its definition. That
visual clue would be enough to signal that this is not just "if" or a typo.