Reference: Abbott, Richard. Analysis and Design of Composite and Metallic Flight Vehicle Structures 3 Edition, 2019

The first moment of area or statical moment is a measure of the distribution of the area of a shape in relation to the axis. The first moment of area is the summation of area multiplied by the distance of the centroid of that area to an axis:

The first moment of area is used to calculate the plastic bending shape factor. It is also used to calculate the shear stress distribution in a cross section. See Section 3.4.1.3.