barn
Areaexact by definition
The barn is exactly 10⁻²⁸ square metres, 100 fm², which is roughly the cross-sectional area of a uranium nucleus. It is the unit of nuclear reaction cross section, a quantity with the dimensions of area that expresses how likely an interaction is rather than how big anything physically is.
Watch out: A cross section is not a physical area. It is a probability expressed in area units, and it varies enormously with neutron energy: uranium-235's fission cross section is around 585 b for thermal neutrons and about 1 b for fast ones, which is the entire reason reactors use moderators.
| 1 b | 1.0000000e-28 m² |
It was coined at Purdue in 1942 during the Manhattan Project, deliberately as an innocuous name that would not reveal its subject in correspondence. The joke is that a nucleus this size is "as big as a barn" to a passing neutron, which is enormous by nuclear standards.