The horizontal distance, directed toward the catcher, from set point to release of the ball from the hand. Extension is not  the equivalent of a pitcher’s stride. 

On-field Impact:

Extension is important for pitchers because it essentially shortens the distance and puts them closer to the plate. As the extension is increased, the batter has less time to recognize and react to the pitch.

Real Life Example:

Assuming a typical pitcher’s extension of 6 feet (for a 40 ft mound position), a 60 mph fastball thrown with an extension of 7.5 feet looks as fast as a 63 mph fastball; whereas, a 60 mph fastball with an extension of 5 feet looks like a 58 mph fastball.