eavesdropper

eavesdrop +‎ -er (“person or thing which does”)

• (UK) IPA^(key): /ˈiːvzˌdɹɒpə(ɹ)/

eavesdropper (plural eavesdroppers)

1. One who eavesdrops.
2. (military, slang) A surveillance aircraft.
   • 1986, Dick Van Der Aart, Aerial Espionage: Secret Intelligence Flights by East and West, page 96:

     […] the Japanese Ministry of Defence rang the bell and informed the world's press about the Tokyo Express — the ironic Japanese nickname for the constant flow of Soviet flying eavesdroppers in Japanese airspace.