Overview
Katyayani (Sanskrit: कात्यायनी, Kātyāyanī, meaning 'she who is of Katya') is a form of the Hindu goddess Mahadevi. In tradition, she is venerated as the slayer of the tyrannical asura Mahisha, and is regarded as one of the principal warrior aspects of the Devi.
Within the framework of the Navadurga, the nine forms of Durga, Katyayani is counted as the sixth. She is accordingly worshipped on the sixth day of the Navaratri festival, which is dedicated to the various manifestations of the goddess across nine nights.
In Shaktism, Katyayani is associated with the fierce forms of Shakti or Durga, alongside such manifestations as Bhadrakali and Chandika. These forms collectively emphasise the martial and protective dimensions of the goddess, who is invoked as a vanquisher of evil and a guardian of cosmic order.
References
Adapted from the English Wikipedia article on Katyayani.