المعنى الاصطلاحي :

That a person makes his faith contingent upon the will of Allah.

الشرح المختصر :

"Istithnaa fil-Imaan" (exception regarding faith) is when a person says: "I am a believer, if Allah wills," or says: "I hope that I am a believer." It has three cases: 1. That a person says it out of his doubt of having faith in principle. This is forbidden and constitutes "kufr" (disbelief), because faith is based upon assertion, which is contrary to doubt. 2. That a person says it out of fear of ascribing virtue to himself and of claiming to have perfect faith. This is permissible. 3. That a person says it out of seeking blessings in mentioning Allah's will, or for the purpose of informing that the faith he has in his heart is by the will of Allah. This is also permissible. "Ahlus-Sunnah wal-Jamaa`ah" make "istithnaa" regarding faith out of their intense fear of Allah and to confirm divine predestination and to negate ascribing virtue to themselves, not out of doubting their faith in what they must believe in. Rather, they fear lest they fall short in realizing true faith, and they hope to fulfill its obligations and perfections. "Istithnaa" may only be with regard to deeds, not with regard to belief in the heart or speech of the tongue, because these last two are decisively known by the person (whether they are true and sincere or not).