A barn is a unit of cross section used in particle physics; it is used to indicate the probability of a reaction between nuclear particles and atomic nuclei or other nuclear particles. One barn is equal to 10^-28 square meters (10^-24 square centimeters).

Many interesting cross sections are measured in pb (picobarns), which are equal to 10^-12 barns. The physical cross sectional area of the nucleus of an atom is approximately 1 barn but depending on the reaction involved the actual reaction cross sectional area can vary over many decades of barns in size and are very dependent on particle energy.