1) a top leader (as in politics)
2) a businessperson of exceptional wealth and power : magnate
Two real estate tycoons—a young New Mexico up-and-comer and an older West Texas powerhouse—are duking it out in the courts over a bungled investment deal that has pitted the former business partners against each other.
— Lauren Villagran, The Albuquerque Journal, 14 June 2016
While tycoon now most often refers to a very wealthy and powerful businessperson, the word has had two other uses in English as well.
When the United States forced Japan to open full commercial and diplomatic relations with the West in 1854, the real ruler of the island nation was the shogun. Officially only a military deputy of the emperor, the shogun—a title shortened from seii-taishōgun, meaning “barbarian-subjugating generalissimo”—stood at the pinnacle of a feudal hierarchy based at Edo (later Tokyo) that effectively controlled the imperial court at Kyoto and ruled the country. Westerners in the initial period of diplomatic relations concluded that the shogun was a sort of secular emperor and the emperor something like the pope. Townsend Harris, the first American consul to Japan, got the idea that the shogun's correct title was taikun, a Japanese borrowing from Middle Chinese elements equivalent to Beijing Chinese dà “great” and jūn “prince.” This word, in the spelling tycoon, became quite popular in America immediately before and during the Civil War as a colloquialism meaning “top leader” or “potentate.” (John Hay, President Lincoln's personal secretary—and later Secretary of State to Presidents McKinley and Theodore Roosevelt—referred to Lincoln as "the Tycoon.") After fading from use for several decades tycoon was revived in 1920s journalism with the narrower sense “a businessman of exceptional wealth and power,” a usage that continues to be part of English.