When used in Buddhist literature, the Sanskrit word vajra usually is defined "diamond" or "adamantine." It can also mean "thunderbolt," although this definition of vajra is more often associated with Hinduism.
A diamond is spotlessly pure and indestructible. As such, the word vajra sometimes signifies enlightenment, or the absolute reality of shunyata, "emptiness."
The vajra also is ritual object associated with Tibetan Buddhism, also called by its Tibetan name, dorje. These objects usually are made of bronze, vary in size and have three, five or nine spokes that usually close at each end in lotus shape. The number of spokes and the way they come together, or not, at the ends have numerous symbolic meanings.
In Tibetan ritual, the vajra often is used together with a bell. The vajra is held in the left hand and represents the male principle, upaya, action or means. The bell is held in the right hand and represents the female principle, prajna, wisdom.
A double dorje, or vishvavajra, are two dorjes connected to form a cross. A double dorje represents the foundation of the physical world and is also associated with certain tantric deities.