The word mole has several meanings:

1. A small, burrowing mammal with velvety fur, tiny eyes, and powerful front paws built for digging. Moles spend most of their lives underground, creating networks of tunnels as they hunt for earthworms and insects. Gardeners often notice their presence from the raised ridges of dirt that appear across lawns. Though moles can't see well, they navigate their dark tunnels using their sensitive snouts and whiskers.
2. A small, dark spot on the skin, usually brown or black. Everyone has moles somewhere on their body. Some people have just a few, while others have dozens. Most moles are harmless and stay the same size throughout your life, though doctors recommend watching for moles that change shape or color.
3. A secret agent who works inside an organization while actually spying for an enemy. A mole in a spy agency might pretend to be loyal while secretly passing information to another country. The term comes from the animal, since both the spy and the creature work hidden from view, tunneling beneath the surface where no one can see them.
4. In chemistry, a unit for measuring amounts of substances, equal to about 602 sextillion particles (such as molecules or atoms). Scientists use moles like bakers use dozens: it's a convenient way to count extremely tiny things in large quantities.