the thunderbolt (vajra, same term as for ‘adamantine’) is the characteristic emblem of a special school of Buddhist doctrine which styles itself Vajrayāna, ‘The Vehicle of the Irresistible Thunderbolt,’ ‘The Way toward the Adamantine Reality of Transcendent Truth.’
Zimmer argues that the thunderbolt (vajra) in Indian and Buddhist thought is not merely a weapon but the supreme symbol of indestructible, transcendent reality, giving its name to an entire doctrinal vehicle.
, Myths and Symbols in Indian Art and Civilization, 1946thesis