mole

Hunting a C.I.A. Mole, Agents Gambled and Let a Suspect Return to China
By MATT APUZZO and ADAM GOLDMAN

Officials hoped it would help solve the mystery of the deaths of C.I.A. informants. Five years later, an arrest has been made but the mystery remains.

https://en.oxforddictionaries.com/definition/mole

 mole

A spy who gradually achieves an important position within the security defences of a country.

‘a well-placed mole was feeding them the names of operatives’

mole4

(also mol)

NOUN

Chemistry 

• 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.

Origin

Early 20th century: from German Mol, from Molekul, from Latin (see molecule).