Complex sentence: The term's historic usage has been in reference to World Series games played between New York teams, but since 1997 it has also been applied to interleague play during the regular season between the American League New York Yankees and National League New York Mets.
Split into simpler sentences: The term's historic usage has been in reference to World Series games played between New York teams. Since 1997, the term Subway Series has also been applied to interleague play during the regular season between the Yankees and National League New York Mets.