Ekayāna is a Sanskrit word that can mean "one path" or "one vehicle". The word took on special significance as a metaphor for a spriritual journey in the Brihadaranyaka Upanishad (II.iv.11 and IV.v.12). Notably, in that text the phrase vedānāṃ vāk ekayānam translates approximately to "the one destination of the Vedas is the spirit of the word".
Read more about Ekayāna: Pali Canon, Indian Mahayana Buddhism, Chinese Buddhism, Japanese Buddhism