The exact reason why the American professional baseball championship is referred to as the World Series is unclear. The term "World's Championship Series" was first used in 1903 to describe the end-of-season nine-game series between the Pittsburgh Pirates and the Boston Red Sox, then known as the Boston Americans. The name has since evolved to simply be the World Series.
The World Series has been an annual sporting event in America almost every year since 1905. A 1994 player strike prevented the game from taking place that year. Though the name seems to imply global participation, Americans seem to implicitly understand that the Series is played by at most two countries, with Canada being represented if the Toronto Blue Jays are playing.