cross-sectional area

A cross section of an object is the intersection of an imaginary plane with that object. In the case of an object whose length is large compared to its other dimensions; an object such as a rod, rope, beam, column, or pipe; the expression "cross section" by itself means the cross section that is perpendicular to the length of the object. Thus the cross-sectional area of a pipe is the area of that circle that is formed by the intersection of an imaginary plane, perpendicular to the length of the pipe, with the pipe.