In baseball circles, a term used to send a short texting. The rule signifies the unwillingness of a spectator/diehard fan to depart from a ballgame before its completion, irrespective of how many innings it may take to do so beyond the standard nine innings of the sport.

Origin of the term is not known, nor the reason for the selection of the number 18, except to simply note the length of two regular games. Potentially, extra innings can extend indefinitely past 18, but it's rare.
A text message by Friend #1 using "18 Innings" Rule:
"Game tonight? 18inn in effect."

Friend #2:
"Can't risk it. Long drive ahead early tomorrow morning... family vacation... but thanks anyway!"

Friend #3:
"I'm game. But will drive separately, just in case."
by Kayrocker October 4, 2009
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