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Definitions of gain line:
noun: (rugby) an imaginary line running across the pitch perpendicular to the touchline and passing through the breakdown or set piece. The subsequent phase is said to have passed "over the gain line" if the gain line at the subsequent breakdown is in front of the last, or "behind the gain line" if it is behind.
noun: (American football) Alternative form of line to gain [(American football) The points on the football field needed to pass to convert a first down.]
