What is "nandaka"

Nandaka (literally "source of joy") or Nandaki is the sword of the Hindu god Vishnu. Even though some scriptures recommend the sword to be depicted in Vishnu's hand, it is generally not depicted in the iconography of Vishnu (who is generally depicted four-armed) and makes a later appearance in the god's sculptures. Nandaka is generally depicted in images where Vishnu is represented with more than four arms. The sword is compared to knowledge in Hindu scriptures.

The Vaishnav (Vishnu-worshipping sect) saints Annamacharya and Pey Azhwar are considered the avatars of Nandaka.