What is Cost Inflation Index?

According to clause v of section 48 of the Income Tax Act 1961, Cost Inflation Index means an index as notified by the Central Government in the Official Gazette, having regard to seventy-five percent of average rise in the Consumer Price Index(urban) for the immediately preceding previous year to such previous year.

source https://www.caclubindia.com/articles/cost-inflation-index-44992.asp