ground rule double: Meaning and Definition of

ground' rule" dou'ble

Pronunciation: [key]
— Baseball. Baseball.
  1. a safe hit ruled for two bases according to the rules of a particular stadium, as when a fly ball bounces once in the outfield and then clears a fence.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.