An Etymological Dictionary of Astronomy and Astrophysics

1. Math: The intersection of a plane with a geometric figure, usually at right angles to an axis of symmetry.
   

2. Physics: A quantity that expresses the effective area that a given particle presents as a target to another incident particle, giving a measure of the probability that the incident particle will induce a particular atomic or nuclear reaction. Also called collision cross section. The cross section has the dimensions of a surface. In nuclear and particle physics, the commonly used units for the cross section are the barn and cm².

See also: In classical mechanics, the cross section for the collision of a point particle with a hard sphere is just be the surface of a section through the middle of the sphere. This explains the name “cross section.” → cross; → section.