noun

• (espionage) An internal spy; a person who involves themself with an enemy organisation, especially an intelligence or governmental organisation, to determine and betray its secrets from within.

noun

• (slang, derogatory, chiefly Australia and New Zealand) A moll, a bitch, a slut.

noun

• (nautical) A massive structure, usually of stone, used as a pier, breakwater or junction between places separated by water.
  "[Alexander the Great] then conceived the stupendous idea of constructing a mole, which should at once connect [Tyre] with the main land; and this was actually accomplished by driving piles and pouring in incalculable quantities of soil and fragments of rock; and it is generally believed, partly on the authority of ancient authors, that the whole ruins of Old Tyre were absorbed in this vast enterprize, and buried in the depths of the sea [...]"
• (rare) A haven or harbour, protected with such a breakwater.
• (historical) An Ancient Roman mausoleum.

noun

• (chemistry, physics) In the International System of Units, the base unit of amount of substance; the amount of substance of a system which contains exactly 6.02214076×10²³ elementary entities (atoms, ions, molecules, etc.). Symbol: mol. The number of atoms is known as Avogadro’s number. [from 1897]