Today it means: A Japanese stock market index; normally refers to the Nikkei 225, which is calculated by aggregating the share prices of 225 large Japanese companies quoted on the Tokyo Stock Exchange, and thus reflects the performance of major Japanese businesses. Full definition →
Borrowed from Japanese 日(にっ)経(けい) (nikkei), an abbreviation of 日(に)本(ほん)経(けい)済(ざい)新(しん)聞(ぶん) (Nihon Keizai Shinbun, literally “Japan Economics Newspaper”), a Japanese newspaper which publishes the index.
Etymology from Wiktionary, CC BY-SA 4.0.