Skt., vajrayana: Diamond Vehicle
Tib., rdo-rje theg-pa
The Diamond Vehicle, a name for the esoteric, tantric form of Buddhism found mainly in Tibet, Bhutan, Nepal, Ladakh and Mongolia; but which has also spread to China and Japan.
The term distinguishes it from Hinayana and Mahayana, although Vajrayana in itself is a special case of the latter. Just as Mahayana (the Greater Vehicle) is an advanced system in relation to Hinayana (the Lesser Vehicle), Vajrayana teachings are a further development of these two. Thus Hinayana is regarded as the "Half Word", Mahayana as the "Whole Word"; with Vajrayana being the "True Word". An alternative, though little used name is mantrayana (Skt., "Vehicle of Mantra"; Tib., sngags-kyi-theg-pa), referring to the school's extensive use of mantras during meditation.
For the last few centuries, the Gelugpa have been the largest and dominant school of Tibetan Buddhism in Tibet itself and are the most prominent in the West; mainly because the Dalai Lama, the traditional spiritual and political leader of Tibet, comes from this background.
Most of the major Vajrayana scriptures are contained in the Kanjur, others remained outside this Tibetan canon yet continue to inspire people worldwide and up to the present time.
In early studies on Tibetan Buddhism by Western authors (especially Germans), Vajrayana has sometimes - rather ignorantly - been called Lamaism. A similar notion applied to Christianity would result in a religion named Priestism or Bishopism.
See also my Revised History of the Development of Vajrayana