What is "kasikavrtti"

The ("the commentary of [composed or used in] Kāśi, i.e. Varanasi") is a commentary on Pāṇini, attributed to Jayaditya and Vamana, composed in c. the 7th century.

It is considered the "fourth great grammar" of Sanskrit, after Pāṇini himself (4th century BC), Patanjali's Mahabhasya (2nd century BC) and Bhartrhari's Vakyapadiya (6th century AD).