adj. Between leagues; often specifically between the two major leagues of American baseball.
The Century Dictionary and Cyclopedia
To combine in a league; engage in joint action.
Word Usage
"• Shandler says: Jeter has ridden his resurgent season into the playoffs (3 HRs, .435 on-base percentage) and lashed the Phillies in interleague play (2 HRs, .462 average)."