The term vajra is a Sanskrit word that is usually defined as "diamond" or "thunderbolt." It also defines a kind of battle club that achieved its name through its reputation for hardness and invincibility.
A diamond is spotlessly pure and indestructible. The Sanskrit word means "unbreakable or impregnable, being durable and eternal". As such, the word vajra sometimes signifies the lighting-bolt power of enlightenment and the absolute, indestructible reality of shunyata, "emptiness."
The vajra as a symbol predates Buddhism and was found in ancient Hinduism. The Hindu rain god Indra, who later evolved into Buddhist Sakra figure, had the thunderbolt as his symbol.
Buddhism integrates the word vajra into many of its legends and practices.The vajra also is a literal ritual object associated with Tibetan Buddhism, also called by its Tibetan name, Dorje. It is the symbol of the Vajrayana school of Buddhism.