barn, abbr. b, in physics, unit of nuclear cross section, i.e., the effective target presented by a nucleus for collisions leading to nuclear reactions; it is equal to 10^-24 square centimeters. The barn is approximately the size of the geometric cross section of an atomic nucleus; the term was coined because an effective cross section that large would present a target "as big as a barn," i.e., an easy target for nuclear bombardment. In practice, effective cross sections of nuclei for many reactions are measured in millibarns (10^-3 barn) because, for most interactions, only a small fraction of collisions cause reactions.

Any of several species of nocturnal birds of prey (genus Tyto), sometimes called monkey-faced owls because of their heart-shaped facial disk and absence of ear tufts. Barn owls are about 12–16 in. (30–40 cm) long, white to gray or yellowish to brownish orange. Their dark eyes are smaller than those of other owls. They hunt mainly small rodents, often on cultivated land, and nest in hollow trees, buildings, towers, and old hawk nests. The common barn owl is found worldwide except in Antarctica and Micronesia. Other species occur only in the Old World.

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Farm building used for sheltering animals, their feed and other supplies, farm machinery, and farm products. Barns are named according to their purpose (e.g., hog barns, dairy barns, tobacco barns, and tractor barns). The principal type in the U.S. is the general-purpose barn, used for housing livestock and for storing hay and grain. Most North American and European farms have one or more barns. They usually consist of two stories, though one-story barns gained in popularity in the late 20th century.

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