The Nikkei 225 was the benchmark index at the center of Japan’s 1989 stock-market peak.
The index reached an intraday high of 38,957.44 on December 29, 1989, near the end of Japan’s asset-price bubble. The bubble involved rapidly rising stock and land prices, abundant credit, and strong optimism about Japan’s economy.
After the peak, asset prices collapsed. The Nikkei fell for years, while banks and companies struggled with bad loans and excessive debt. The prolonged economic weakness became associated with Japan’s “Lost Decades.”
The TOPIX is another important Japanese stock index, so it is a plausible alternative but not the answer to this question. The Nikkei 225 is price-weighted and tracks 225 prominent companies listed on the Tokyo Stock Exchange’s Prime Market.