Ekayāna is a Sanskrit word that means ‘One Vehicle’ or ‘Single Vehicle’. This idea features prominently in Tendai (Tientai) doctrine and comes from the Lotus Sutra. Buddhism is made up of many different ‘vehicles’ or ‘yanas’ – Śravakayāna, Mahayāna, Vajrayāna etc. The Ekayāna principle claims that all these different paths or ‘vehicles’ are expedient means (Upaya Skt; Hoben 方便 Jp), or devises used in order to teach and appeal to all the myriad sentient beings of the cosmos. This principle is found in the Lotus Sutra, most prominently in the Burning House Parable (Chapter 3). This principle was also used by Chih-i to explain and catagorize the countless (and often confusing and contradicting) Buddhist Sutras in his 5 Periods and 8 Teachings thesis.
What the Ekayāna principle teaches us, above everything, is that all the different schools are a means to an end and not the end itself. Whether you follow the Śravakayāna, Mahayāna, or Vajrayāna, you are following the Buddha-dharma.
Tendai Buddhism UK 