Bayous are either stagnant or slow-moving bodies of water which fill abandoned waterways. The term bayou by itself usually refers to a very slow stream, while the term bayou lake is often used to describe lakes or ponds in similar conditions. A bayou is normally found in very flat regions, as the lack of a slope reduces the speed at which the water flows. A bayou is usually the outflow from a nearby major waterway, such as the Mississippi river, and as such may be affected by flooding in the main waterway.

The term bayou is used primarily in the southern United States. The term slough is also used to describe essentially the same geographical feature, although it tends to have a more widespread usage. The bayou is particularly associated with the Cajun culture of Louisiana, and parts of Texas and Alabama. Bayou are also found throughout Arkansas, although most in that state use an alternate spelling of buyou.

Because of its slow-moving nature, and the relative lack of differentiation in height, a bayou often forms a sort of bog or marsh. In fact, the easiest way to think of a bayou is simply as a stream moving through a swamp. Plants and trees may grow up through the bayou, making the water seem to flow haphazardly through the land as a whole. The ecology in bayou wetlands is quite diverse, and in some cases unique. The land in a bayou is always damp, leading to specialized plants that do well with little or no oxygen, and which are equipped to make use of the copious water. Cattails, water lilies, bulrushes, and cedar are all examples of vegetation which does well in the bayou environment.

Because of its often mazelike character, eternal dampness, and sometimes haunting vegetation, the bayou is often the location in fiction of frightening stories. Tales of voodoo often take place in the bayou, and a number of horror stories are set in the deep bayou, far from civilization. Alligators inhabit much of the bayou in Louisiana, adding to the mystique of fear that surrounds the region.

The city of Houston, Texas is often referred to as Bayou City. This is a reference to the many streams that make their way through the city of Houston, or just outside the city limits. The White Oak Bayou, Buffalo Bayou, and Carpenters Bayou are the most famous of these bayous, but there are more than eighteen bayous in the region.