(Science: physics) Usually refers to the (apparent) area presented by a target particle to an oncoming particle (or electromagnetic wave). This measures the probability of an interaction occuring. For typical interactions between ions and electrons, or between two nuclei, these cross sections are generally measured in barns.
(Science: anatomy) a transverse cut through a structure or tissue. The opposite of a cross-section is a longitudinal section. By analogy, a study may be cross-sectional or longitudinal.