an agricultural building with large, usually sliding doors, prominent roofs, and predominantly open spaces on the interior, primarily used as storage buildings for hay, grains, and farm equipment and shelters for livestock.
A barn is an agricultural building used for storage and as a covered workplace. It may sometimes be used to house animals or to store farming vehicles and equipment. Barns are most commonly found on a farm or former farm.
A barn (symbol b) is a unit of area. While the barn is not an SI unit, it is accepted (although discouraged) for use with the SI. Originally used in nuclear physics for expressing the cross sectional area of nuclei and nuclear reactions, today it is used in all fields of high energy physics to express the cross sections of any scattering process.