The Nikkei is one of the world’s most well-known stock indices, and is considered a barometer of the Japanese economy. It is one of the three major stock indices in Japan, alongside the Tokyo Price Index (TOPIX) and the Mothers Market Index, which tracks start-up companies listed on the Tokyo Stock Exchange’s Mothers Market. The Nikkei is comprised of the country's top 225 blue-chip stocks, which are selected based on criteria such as market capitalization, stock price, liquidity and profitability.

The Nikkei index is price weighted, meaning that the index is an average of the share prices of the companies listed. Companies that have the highest share prices carry the most weight in the index. Companies listed on the Nikkei are some of the biggest and most influential companies in Japan, such as Sony Corporation, Canon Inc, Nissan Motor Company and Honda Motor Company.

Unlike many other stock indices, the Nikkei does not include dividends in its calculation. Instead, it is calculated by summing up the share price of all the companies in the index and dividing it by the number of companies in the index. Index changes are based on market reactions to news and events, with the listed companies' share prices either rising or falling in response.

The performance of the Nikkei affects the performance of many of the Japanese funds held by investors around the world. The Nikkei is also used to help determine the value of the Japanese yen and other Asian currencies, as it is considered to be a relatively stable stock index.

The Nikkei is considered to be one of the most stable and well-known stock indices in the world, and is often seen as a gauge of the overall performance of the Japanese economy. It is an important benchmark for investors looking to tap into the Japanese stock markets,and its performance can provide investors with insight into the performance of individual stocks and the overall economy in Japan.