kihō ittai (Japanese). A Japanese Pure Land doctrinal concept that the Buddha Amitābha and the believer who recites the nembutsu are of one substance. This idea accounts for the efficacy of the practice by identifying the very Buddhahood of Amitābha with the assurance of the believer's rebirth in the Pure Land; without this direct link, the Buddha would not be a Buddha because his vow (that all believers who call upon him would achieve rebirth or else he would not accept Buddhahood) would not be fulfilled, and without the fulfilment of this vow, the believer could not attain rebirth in the Pure Land of Sukhāvatī. This idea originated in the literature of the Jōdo Shū, but later became common in the Jōdo Shinshū and the Jishū as well.

A Dictionary of Buddhism, Oxford University Press, 2003, 2004 (which is available in electronic version from answer.com)