mole1 / mōl/ • n. 1. a small burrowing insectivorous mammal (family Talpidae) with dark velvety fur, a long muzzle, and very small eyes. Its several species include the eastern mole (Scalopus aquaticus) of North America. 2. a spy who achieves over a long period an important position within the security defenses of a country. ∎ someone within an organization who anonymously betrays confidential information.mole2 / mōl/ • n. a small, often slightly raised blemish on the skin made dark by a high concentration of melanin.mole3 / mōl/ • n. a large solid structure on a shore serving as a pier, breakwater, or causeway. ∎ a harbor formed or protected by such a structure.mole4 / mōl/ • n. Chem. the SI unit of amount of substance, equal to the quantity containing as many elementary units as there are atoms in 0.012 kg of carbon-12.mole5 / mōl/ • n. Med. an abnormal mass of tissue in the uterus. See also hydatidiform mole.