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."

A pen (Latin penna, feather) is a device used to apply ink to a surface, usually paper, for writing or drawing. Historically, reed pens, quill pens, and dip pens were used in ancient period, with a nib of some sort to be dipped in the ink. Ruling pens allow precise adjustment of line width, and still find a few specialized uses, but technical pens such as the Rapidograph are more commonly used. Modern types also include ballpoint, rollerball, fountain, and felt or ceramic tip pens.

The main modern types of pens can be categorized by the kind of writing tip or point:

These historic types of pens are no longer in common use:

Ancient Indians were the first to use the pen. According to ancient text the earliest of pens made in India used bird feathers, bamboo sticks, etc. The old literature of Puranas, Ramayana and Mahabharta used this kind of pen roughly 500 BC.^{[citation needed]}Ancient Egyptians had developed writing on papyrus scrolls when scribes used thin reed brushes or reed pens from the Juncus maritimus or sea rush. In his book A History of Writing, Steven Roger Fischer suggests that on the basis of finds at Saqqara, the reed pen might well have been used for writing on parchment as long ago as the First Dynasty or about 3000 BC. Reed pens continued to be used until the Middle Ages although they were slowly replaced by quills from about the 7th century. The reed pen, generally made from bamboo, is still used in some parts of Pakistan by young students and is used to write on small boards made of timber.^{[citation needed]}

In English social and economic history, enclosure or inclosure is the process which ends traditional rights such as mowing meadows for hay, or grazing livestock on common land formerly held in the open field system. Once enclosed, these uses of the land become restricted to the owner, and it ceases to be common land. In England and Wales the term is also used for the process that ended the ancient system of arable farming in open fields. Under enclosure, such land is fenced (enclosed) and deeded or entitled to one or more owners. The process of enclosure began to be a widespread feature of the English agricultural landscape during the 16th century. By the 19th century, unenclosed commons had become largely restricted to rough pasture in mountainous areas and to relatively small parts of the lowlands.

The process of enclosure has sometimes been accompanied by force, resistance, and bloodshed, and remains among the most controversial areas of agricultural and economic history in England. Marxist and neo-Marxist historians argue that rich landowners used their control of state processes to appropriate public land for their private benefit. This created a landless working class that provided the labour required in the new industries developing in the north of England. For example: "In agriculture the years between 1760 and 1820 are the years of wholesale enclosure in which, in village after village, common rights are lost". "Enclosure (when all the sophistications are allowed for) was a plain enough case of class robbery".