Vajrayāna

(Sanskrit, diamond vehicle). A late Indian term used to designate the path (yāna) of tantric Buddhism. The name is derived from the image of the thunderbolt (vajra) which was used to symbolize the imperishable nature of enlightenment (bodhi), the indivisibility of appearances and emptiness (śūnyatā) or of compassion (karuṇā) and insight (prajñā).

A Dictionary of Buddhism, Oxford University Press, 2003, 2004 (which is available in electronic version from answer.com)