eavesdropper

What it means

1. One who eavesdrops.
2. slanggovernmentA surveillance aircraft.

   […] 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.

   1986, Dick Van Der Aart, Aerial Espionage: Secret Intelligence Flights by East and West, page 96

In a sentence

[…] 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.

1986, Dick Van Der Aart, Aerial Espionage: Secret Intelligence Flights by East and West, page 96

Where it comes from

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