In physics, spin is the velocity of rotation of something around a particular axis. Spin is sometimes called angular momentum, which is defined as:
(mass) x (velocity) x (radius), where radius is the distance from the spinning object to the axis.
Conservation of angular momentum, one of the fundamental laws of physics, observes that the angular rotation of a spinning object remains constant unless acted upon by external torque.
The underlying principle can be illustrated by the spin of a figure skater with outflung arms, considering the spinning object the skater's hands and the body the axis. Velocity is inversely proportional to the radius, increasing if the radius becomes smaller and decreasing if it becomes larger. Similarly, the distance of the skater's hands from his body controls the speed of the spin: As the skater brings the arms in closer to the body (at a smaller angle relative to the body), the speed of the spin increases and as he moves them farther out (at a greater angle relative to the body), the speed of the spin decreases.
Spin of subatomic particles is measured in multiples of a unit called the Dirac h, or h-bar.
Gyroscopes exploit the conservation of angular momentum to stabilize, guide or measure rotational movement in many types of systems. Wheels on a bicycle, for example, act as gyroscopes as they spin up to speed, making it easier to stay upright and harder to upset momentum.
Khan Academy explores angular momentum:
In public relations and journalism, spin is the selective assembly of fact and nuance to support a particular view of a story.