The quantity mir2i is the second moment of the i th mass with respect to (or "about") the axis, and the sum ∑mir2i is the second moment of mass of all the mas...The quantity mir2i is the second moment of the i th mass with respect to (or "about") the axis, and the sum ∑mir2i is the second moment of mass of all the masses with respect to the axis. But the question should be asked: "What is the purpose of calculating the squares of the distances of lots of particles from an axis, multiplying these squares by the mass of each, and adding them all together?