A pen is an enclosure for holding livestock. The term describes multiple types of enclosures that may confine one or many animals. Construction and terminology varies depending on region of the world, purpose, animal species to be confined, local materials used, and cultural tradition. "Pen" or "penning" as a verb refers to the act of confining animals in an enclosure.

In Australia and New Zealand a pen is a small enclosure for livestock (especially sheep or cattle), which is part of a larger construction, e.g. calf pen, forcing pen (or yard) in sheep or cattle yards, or a sweating pen or catching pen in a shearing shed. In Australia, a "paddock" may encompass a large, fenced grazing area of many acres, not to be confused with the American English use of "paddock" as interchangeable with "corral" or "pen," describing smaller, confined areas.

In the United States, the term "pen" usually describes small enclosures for holding sheep, goats, and pigs. A pen for cattle is also sometimes called a corral. Pens may be named by their purpose, such as a "holding pen," used for short-term confinement. Groups of pens that are part of a larger complex may be called a "stockyard," where a series of pens hold a large number of animals, or a "feedlot," which is type of stockyard used to confine animals that are being fattened. A large pen for horses is called a paddock (Eastern USA) or a corral (Western USA), a borrowing from the Spanish language. In some places an exhibition arena may be called a "show pen." A small pen for horses (no more than 15–20 feet on any side) is a "pen" if it lacks any roof or shelter, otherwise it is called a "stall" and is part of a stable. A large fenced grazing area of many acres is called a "pasture," or, in some cases, "rangeland."