1. A plane surface formed by cutting a solid, especially by cutting at right angles to its longest axis.

2. The area of such a surface.

3. A measure of the probability that a collision will occur between a beam of radiation and a particular particle, expressed as the effective area presented by the particle in that particular process. It is measured in square metres or barns. The elastic cross section accounts for all elastic scattering in which the radiation loses no energy to the particle. The inelastic cross section accounts for all other collisions. It is further subdivided to account for specific interactions, such as the absorption cross section, fission cross section, ionization cross section, etc.