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A barn (symbol: b) is a unit of area equal to 10−28 m2 (100 fm2). Originally used in nuclear physics for expressing the cross sectional area of nuclei and nuclear reactions, today it is also used in all fields of high-energy physics to express the cross sections of any scattering process, and is best understood as a measure of the probability of interaction between small particles.
The square mile (abbreviated as sq mi and sometimes as mi²) is an imperial and US unit of measure for an area equal to the area of a square with a side length of one statute mile.