What is the name of Vishnu's discus weapon?

Vishnu’s discus weapon is called the Sudarshana Chakra.

The Sudarshana Chakra is portrayed as a divine, spinning discus and is one of Vishnu’s principal attributes. In images of the four-armed deity, it is commonly held in the rear right hand. Vishnu’s other familiar emblems are the Panchajanya conch, the Kaumodaki mace, and the lotus.

The name combines Sanskrit terms commonly understood as referring to auspicious or excellent vision and a wheel or discus. Beyond being a weapon, the chakra can symbolize cosmic order, time, and Vishnu’s power to destroy forces that threaten dharma. In epic and Puranic stories, it is used against demons and other enemies of the gods.

Krishna, who is identified with Vishnu in many traditions, also wields the Sudarshana Chakra. A famous Mahabharata episode describes Krishna using it to kill Shishupala, while another story says he employs it to create the appearance of sunset during the Kurukshetra War. It is not the Trishula, which belongs to Shiva, or the Vajra, associated with Indra.