spin

Spin is an intrinsic angular momentum that a particle carries simply by being what it is, the way it carries mass or charge. An electron has spin even when it is sitting perfectly still, and nothing you do can make it spin faster or slower or stop. It is a fixed, built-in property, not a motion you can speed up or wind down.

The name is borrowed from a tempting picture: a tiny ball whirling on its axis. That picture is genuinely useful for intuition, but taken literally it fails. An electron is, as far as every experiment can tell, a point with no size, so there is no surface to go around and no honest sense in which something is rotating. Spin behaves mathematically like angular momentum and adds to orbital angular momentum, yet it has no classical counterpart at all.

Spin matters because it quietly shapes the everyday world. It is why electrons stack into atomic shells instead of all collapsing to the lowest level, which gives the periodic table its structure; it is why magnets are magnetic; and it is the resource manipulated in magnetic resonance imaging and in many quantum computers. A property with no classical picture turns out to underpin chemistry, magnetism, and modern technology.