Definition of "iff"

iff¹

/ɪf ən(d) ˈəʊn.lɪ ɪf/

conjunction
1. (mathematics, logic) Short for if and only if.

iff²

/ɪf ən(d) ˈəʊn.lɪ ɪf/

conjunction
1. Obsolete spelling of if.
  Quotations
  For iff the patient be aſhamed to diſcloſe hiſ wounde to the phyſician which knoweth it not , phyſicke can not helpe him.
  1565, Thomas Stapleton, A Fortreſſe of the Faith &c., Ihon Laet, the 2. part. 112