The Collaborative International Dictionary of English v.0.48:

cross section \cross section\ n.
   1. a flat plane cutting through a three-dimensional object,
      usually at right angles to the longest axis of the object.
      [PJC]

   2. any visual representation of a cross section[1], showing
      the internal structure of the object in the plane of the
      cross section; as, the technician prepared a series of MRI
      cross sections of the skull.

   Note: Different cross sections created by different
         techniques may show different aspects of internal
         structure. Thus computerized axial tomography using
         X-rays shows different structures than are visualized
         by MRI.
         [PJC]

   3. a thin slice of an object made by cutting it transversely;
      as, to view a cross section of a bacterium with an
      electron microscope after staining the DNA; cross sections
      were prepared with a microtome.
      [PJC]

   4. a representative sample of a complex group; as, the town
      contained a cross section of the American population.
      [PJC]

   5. (Physics) a measure of the probability that a nucleus will
      interact in a specified way with a bombarding particle,
      expressed as the effective area that the nucleus presents
      to the particle; -- called also nuclear cross section.
      [PJC]