H. P. Blavatsky

Vajra. Lit., “diamond club” or sceptre. In the Hindu works, the sceptre of Indra, similar to the thunderbolts of Zeus, with which this deity, as the god of thunder, slays his enemies. But in mystical Buddhism, the magic sceptre of Priest‐Initiates, exorcists and adepts — the symbol of the possession of Siddhis or superhuman powers, wielded during certain ceremonies by the priests and theurgists. It is also the symbol of Buddha’s power over evil spirits or elementals. The possessors of this wand are called Vajrapâni (q.v.).

Source: H. P. Blavatsky, The Theosophical Glossary, art. Vajra