The Soka Gakkai Dictionary of Buddhism
one vehicle[一乗] (Skt eka-yana; Jpn ichijo )
Also, single vehicle, Buddha vehicle, one Buddha vehicle, one vehicle of Buddhahood, or supreme vehicle. The teaching that leads all people to Buddhahood. It is taught in the Flower Garland Sutra and other Mahayana sutras, but the Lotus Sutra places the greatest emphasis on it. The Buddha's teaching is compared to a vehicle (Skt yana) that carries one to a particular state of enlightenment. In accordance with people's capacities, the pre-Lotus Sutra teachings expound and emphasize the voice-hearer vehicle (shravaka-yana), which leads one to the state of arhat; the cause-awakened one vehicle (pratyekabuddha-yana), which leads one to the state of pratyekabuddha; and the bodhisattva vehicle (bodhisattva-yana), which after many kalpas of practice leads one to Buddhahood. The voice-hearer vehicle and the cause-awakened one vehicle are together termed the two vehicles, and with the addition of the bodhisattva vehicle, the three vehicles.The Lotus Sutra teaches that these three vehicles are not ends in themselves but means to lead people to the one vehicle, which unifies and refines the three vehicle teachings. The "Expedient Means" (second) chapter of the sutra says that the Buddhas employ only a single vehicle to preach the Law to living beings. It also says that the Buddhas, utilizing the power of expedient means, divide the one vehicle and preach as though it were three. The chapter again says that there is only one vehicle in all the Buddha lands throughout the universe, and the Buddha's sole purpose is to lead all beings to Buddhahood. The T'ient'ai school called this the "replacement of the three vehicles with the one vehicle." In the Lotus Sutra, the term one vehicle is synonymous with the Buddha's true teaching.