\ˈif-ᵊn(d)-ˈōn-lē-ˌif; ˈif, sometimes read with a prolonged f \

Definition of iff 

(Entry 1 of 2)

: if and only if two figures are congruent iff one can be placed over the other so that they coincide used in logic and mathematics

Definition of IFF (Entry 2 of 2)

identification, friend or foe

First Known Use of iff

Conjunction

1955, in the meaning defined above

History and Etymology for iff

Conjunction

alteration of if entry 1

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The first known use of iff was in 1955

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