This is a baseball term which describes a facet of the schedule in Major League Baseball in which a team from the National League (NL) plays a team from the American League (AL) in a regular season game. Prior to 1997, teams only played other teams within their league. The World Series is where the National League champion would play the American League champion. Interleague play was introduced in order to introduce variety in the schedule and ignite local rivalries. Games are organized by division, with teams in one division of one league playing all the teams in the other division. The local rivalries come into play when, for example, teams from the AL East play teams from the NL East. These games are typically scheduled to take place after the All-Star game, with most games coming in May or June.
The rules of interleague play dictate that home team's league's rules are to be followed. The clearest example of this is the designated hitter rule, which only exists in the American League. If the games are being played on the AL team's field, the rule is in effect, while in an NL park, the rule is not in effect. The concept of interleague play has existed for a long time, and was formally proposed to the league in 1958. It finally came into existence in 1997 to help with the after effects of the MLB strike in 1994 that had decimated interest in baseball, and make way for more teams in the playoffs since the league had expanded to 30 total teams.