A word that describes the following machine

[morning]

Hello, I am looking for a word(s) for my composition class. How do you call the following machine:

The machine/small gadget that is usually hidden (e.g. in a place or on someone's clothes) and used to eavesdrop on someone’s conversation.

Please help. Thank you!

[rogusx]

A bug.

[morning]

Thanks for the help. ^_^
I thought that was a slang, and just checked the dictionary and found this is exactly what i want.

Oh, can I just confirm that 'bug' here can be used either as a noun or a verb?

Thanks!

[Tdol]

Noun and verb.