A person is a bachelor IFF that person is a marriageable man who has never married.
British Dictionary definitions for IFF
(logic) a shortened form of if and only if: it indicates that the two sentences so connected are necessary and sufficient conditions for one another. Usually iff is used for equivalence in the metalanguage, rather than as the biconditional in the object language
Identification, Friend or Foe: a system using radar transmissions to which equipment carried by friendly forces automatically responds with a precoded signal