Distance from the parallel plane of the hosel leading edge to the leading edge of the face.
Why Offset?
It's all about getting the club square at impact with the ball. When you hit with an offset club, your hands are slightly ahead of the ball, which can help you lead the clubhead through the impact.
Can offset clubs help my game?
An offset club can help your game if you 1) scoop or hit up on the ball (thereby hitting the ball too high) or if 2) you slice the ball. It's best to work with a professional club-fitter to see exactly how much offset is beneficial for you.
How much offset does Alpha irons have?
Most of the Alpha irons have minimal or no offset. The Alpha Response iron however is designed with up to 6 3/4 mm offset for the #5 iron.