Nikkei index, theNik‧kei in‧dex, the /ˌnɪkeɪ ˈɪndeks/ (also the Nikkei, the ˌNikkei ˈAverage)a number that shows how well or badly shares have performed on the Tokyo stock exchange on a particular day. The number is based on the share prices of about 200 important companies. →Dow Jones Average, FT 100 Share Index, Hang Seng Index