Definition: "Moment of intertia" is the term applied to a clubhead's resistance to twisting when the ball is struck. For example, your swing is a little off and you hit the ball on the toe of the clubhead. A clubhead with a higher MOI will twist less as a result of the mis-hit, creating a better chance that the ball will still go where you intended and with less loss of distance.
Moment of Intertia is a physical property that can be expressed as a numerical measurement. That's not a number that is common to see (yet) in golf clubs. However, the term "moment of inertia" or "MOI" is becoming much more common in advertising and marketing by the golf club companies.
The growth in popularity of term is directly related to the growth in popularity of ever-larger clubheads, both for drivers and putters. The size and weighting properties of a clubhead can be manipulated to add a greater MOI. High-MOI is an important component in clubs that are described as "forgiving" or "game improvement."
For a more in-depth discussion of moment of inertia, see the FAQ, "What is MOI?"
Also Known As: MOI, M.O.I.
