A section done by a plane along the long axis of a structure
The term longitudinal section pertains to a section done done by a plane along the long axis of a structure in contrast to the other term, cross section, which is a section that is cut transversely. Longitudinal sections would therefore pertain to any vertical section such as median, sagittal, and coronal sections.
A sagittal section is a cut done by a plane from the top to the bottom of the body, or of any anatomic structure, dividing it into left and right portions. A medial section is one that which is cut at or towards the central plane, especially of a bilaterally symmetrical organism or structure. A coronal section is one that is cut at any vertical plane that divides the body into ventral and dorsal sections. It is a longitudinal section because it is perpendicular to the transverse section.
An example of a longitudinal section of a body would be at the mid-coronal plane transecting a standing body into two halves, i.e. front (ventral) and back (dorsal), or anterior and posterior.