iff

Short definition from Wiktionary

Conjunction

1. 1
   Abbreviation of if and only if.
2. 2
   Obsolete spelling of if.

Proper noun

1. 1
   Initialism of Interchange File Format, a file format standard developed for Amiga computers.
2. 2
   Initialism of International Flavors & Fragrances

Noun

1. 1
   Initialism of identification friend or foe.