NOUN (3)

1. a section created by a plane cutting a solid perpendicular to its longest axis;

2. a sample meant to be representative of a whole population;

3. (physics) the probability that a particular interaction (as capture or ionization) will take place between particles; measured in barns;

The Collaborative International Dictionary of English v.0.48:

cross section \cross section\ v. t.
   to create one or a series of cross sections[3] by cutting
   (an object) into thin slices.
   [PJC]

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]

WordNet (r) 3.0 (2006):

cross section
    n 1: a section created by a plane cutting a solid perpendicular
         to its longest axis
    2: a sample meant to be representative of a whole population
    3: (physics) the probability that a particular interaction (as
       capture or ionization) will take place between particles;
       measured in barns

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "cross section":
   adjunct, chiasma, component, contingent, crossing, cruciation,
   decussation, detachment, detail, division, dole, embodiment,
   example, exponent, fraction, installment, intercrossing,
   intersecting, intersection, item, parcel, part, particular,
   percentage, portion, quadrant, quarter, quota, random sample,
   remainder, representation, representative, sample, sampling,
   section, sector, segment, share, specimen, subdivision, subgroup,
   subspecies, transection, transversion, traversal, type,
   typification