Vajra

[Sanskrit] Thunderbolt or diamond; that which is hard, impenetrable; that which destroys but is itself indestructible; a scepterlike symbol of the thunderbolt, representing the adamantine nature of Truth, used in rituals, especially of exorcism of demons. The “vajra” is thought to cleave through ignorance and therefore symbolizes the indestructible nature of the Buddha’s wisdom and the victory of knowledge over illusion.

A vajra

In Vajrayana Buddhism, it is the symbol of bodhicitta, or enlightenment. In some traditions, the vajra signifies the union of man and the Buddha; one end of the vajra symbolizes the macrocosmic realm of the Buddha and the other end the microcosmic realm of man.

Sources

Pearls of Wisdom, vol. 31, no. 21, May 22, 1988.