In usage in the Southern United States, a bayou (/ˈbaɪ.uː, ˈbaɪ.oʊ/ ^ⓘ)^[1] is a body of water typically found in a flat, low-lying area. It may refer to an extremely slow-moving stream (often with a poorly defined shoreline), marshy lake, or wetland. They typically contain brackish water highly conducive to fish life and plankton. Bayous are commonly found in the Gulf Coast region of the southern United States, especially in the Mississippi River Delta, though they also exist elsewhere.

A bayou is often an anabranch or minor braid of a braided channel that is slower than the main stem, usually becoming boggy and stagnant. Though fauna varies by region, many bayous are home to crawfish, certain species of shrimp, other shellfish, and leeches, catfish, frogs, toads, salamanders, newts, American alligators, turtles, and snakes such as watersnakes, swampsnakes, mudsnakes, crayfish snakes, and cottonmouths. Common birds include anhingas, egrets, herons, spoonbills, as well as many other species.