n.1. A large building for sheltering livestock, storing hay or other agricultural products, or housing equipment used for operating a farm.
2. A large shed for the housing of vehicles, such as railroad cars.
3. A particularly large, typically bare building: lived in a barn of a country house.
4. Abbr. b Physics A unit of area equal to 10-24 square centimeters, used to measure cross sections in nuclear physics.