A monopod, also called a unipod, is a single staff or pole used to help support cameras, video cameras, binoculars, rifles or other precision instruments in the field.
The monopod allows a still camera to be held steadier, allowing the photographer to take sharp pictures at slower shutter speeds, and/or with longer focal length lenses. In the case of video, it reduces camera shake, and therefore most of the resulting small random movements. When used by itself, it eliminates camera shake in the vertical plane. When used in combination with leaning against a large object, a bipod is formed; this can also eliminate horizontal motion.
Unlike a tripod, monopods cannot support a camera independently. In the case of still cameras, this limits the shutter speed that can be used. They still allow lower shutter speeds than hand holding, and are easier to carry and use than a tripod.
Many monopods can also be used as a "chestpod," or "beltpod," meaning that the foot of the monopod (sometimes with a special adapter) can rest on the belt, waist, or chest, of the photographer. The result is that the camera is held more steadily than by hand alone (though not as steadily as when the foot is planted on the ground), and the camera/monopod is completely mobile, travelling with the photographer's movements. This is similar to a finnstick.
Monopods are usually made to fold or "telescope" quickly when not in use, allowing them to be transported and stored more easily.
Generally, in terms of mobility versus stability, if mobility increases, stability decreases as follows:
Tripod or tablepod resting on a solid surface (most stable)
Monopod on solid surface
Hand-held (least stable)
Monopods are often equipped with a ball swivel, allowing some freedom to pan and tilt the camera while the monopod remains relatively stationary.
Walking sticks or "trekking poles" exist that have a 1/4"-20 threaded stud on the top of the handle, usually covered by a cap when not in use, allowing them to double as a camera monopod. The user would usually need to carry a ball swivel adapter separately and mount it as needed.
Monopods used with a smartphone or camera to take selfie photographs beyond the normal reach of the arm are known as selfie sticks.