In Pure Land Buddhism, the Original Vow, or Fundamental Vow (本願, hongan) refers to a forty eight part vow that Amitābha Buddha made (when he was a bodhisattva long ago). The term is often used to refer solely to the 18th vow in particular, which is very important for Pure Land Buddhist doctrine, especially in Japanese Buddhism.

The term comes from the Sanskrit pūrva-praṇidhāna, which means Past vow or Previous Vow. This term originally referred to any bodhisattva vows made by any bodhisattva in a past life, which retain a special spiritual power even after Buddhahood. Thus, the term can also refer to the past vows of other figures, such as is the twelve vows of Medicine Master Buddha. In Pure Land Buddhism however, the term specifically refers to the vow of Amitābha Buddha (Jp: Amida Butsu) found in the Infinite Life Sutra.

The term Hongan is also the namesake of the Temple of the Original Vow (Hongan-ji), a temple in Kyoto which is also the head temple of the largest sub-sect of Jōdo Shinshū Buddhism.

The text of the 18th vow of Amitabha Buddha, according to Infinite Life Sutra, reads:^[1]

If, when I attain Buddhahood, sentient beings in the lands of the ten quarters who sincerely and joyfully entrust themselves to me, desire to be born in my land, and call my Name, even ten times, should not be born there, may I not attain perfect Enlightenment. Excluded, however, are those who commit the five gravest offences and abuse the right Dharma.^[2]

In the Amitāyurdhyāna Sūtra, Buddha taught Ajātasattu's mother, Queen Videhi, that those who attain birth on the lowest level of the lowest grade are the sentient beings who commit such evils as the five gravest offenses, the ten evil acts and all kinds of immorality, when he is about to die, he may meet a good teacher, who consoles him in various ways, teaching him the wonderful Dharma and urging him to be mindful of the Buddha; but he is too tormented by pain to do so, so the good teacher then advises him to repeat nianfo ten times. Because he calls the Buddha's name, with each repetition, the evil karma which he has committed during eighty kotis of kalpas of Samsara is extinguished. ^[3]

The Japanese term hongan (本願) is derived originally from the Sanskrit term pūrvapraṇidhāna, meaning "past vow" or "previous resolution." However, the use of pūrvapraṇidhāna for the 18th vow of Amitābha is not known in Indian Buddhism, which does not treat the 18th vow in any special way. The term pūrvapraṇidhāna typically refers to all the original vows made by a bodhisattva in order to develop bodhicitta and begin striving toward Buddhahood. Regarding Pure Land practice in Indian Buddhism, Hajime Nakamura writes that as described in the Pure Land sūtras from India, buddhānusmṛti or "mindfulness of the Buddha" is the essential practice.^[4] Buddhānusmṛti is called nianfo in Chinese and nembutsu in Japanese.

In Japanese Buddhism, Amitabha Buddha is often associated with devotional practices, and he is regarded as striving to save those beings who are incapable of reaching Enlightenment through their negative karma, by leading them to Enlightenment. The founder of Jōdo-shū, Hōnen, emphasized the importance of the Primal Vow over the efficacy of practices espoused by his contemporaries in Tendai. These same teachings became central to the later Jōdo Shinshū sect as well.^[5]

This vow forms the basis of Pure Land Buddhism as well as nianfo/nembutsu. As in the vow, it applies not only to a dying person, death of self, but also to an animal,^[6] or a ghost wandering or in hell,^{[7][8][9][10][11][12][13][14][15][16][17][18]} if he has accumulated enough merits in present or past lives,^{[19][20][21][22]} and willing to go there.

• Divine grace