An eavesdropping device was a piece of technology designed to listen into conversations.
In 2372, Quark planted a Ferengi eavesdropping device in a bowl of fruit in an attempt to listen into First Minister Shakaar Edon's conversations, during his visit to Deep Space 9. However, Constable Odo found the device while running security scans of the food. Quark explained that his brother had a medical condition where he sometimes stopped breathing during the night, and claimed that he used the device to monitor him. He said he had recently misplaced the device and was glad he had "found" it. Odo proceeded to destroy the device by dropping it into a bowl of punch, shorting it out. (DS9: "Crossfire")