These words all refer to the action of listening-in on or prying into the private conversations or affairs of others. To overhear is to listen to something being said without the knowledge or intention of the speaker or speakers. Overhearing is the most innocent action in this group because it is accidental in nature. The only time the word suggests a lack of innocence is when we know the overhearing mentioned went on for so long a time that the accident turned into a design. "I couldn’t help overhearing what you said to so-and-so" is often a euphemistic way of saying "I caught part of your private conversation and stayed on to enjoy it." The word for which overhear would be the euphemism in this case is eavesdrop . To eavesdrop is to listen in secret to some kind of private exchange. Unlike overhearing , eavesdropping is never unintentional. Even if one were to blunder into a situation where one overhears something private, the overhearing does not turn into eavesdropping without thought and decision: I sat in a café cubicle next tot he one where Marge and Jim were dining and before long I was eavesdropping on their conversation.

To eavesdrop by means of an electronic receiver is to monitor . The receiver may be planted in a telephone, in the wall of a room, in a chandelier, etc., and is designed particularly to listen-in on conversation with military, political, business or criminal significance. Monitoring is also the word used to designate a kind of observation or surveillance kept by a person or by a camera: Police monitored the suspect’s comings and goings with a camera planted in a building across the street from his home.

To bug is to put a hidden listening device in a room, telephone circuit, etc. the device, which is called a bug , can be so small that it might be concealed as part of a person’s clothing or jewellery. Jokes and cartoons have illustrated the fact that even so harmless-looking a thing as the olive in a martini might be bugged to facilitate a planned invasion of privacy.

To tap or wiretap is to cut into a telephone or telegraph circuit for the purpose of secretly intercepting conversations and messages.

• All conversations having to do with the planned merger of the two companies were wiretapped by a competitor; She hired a private detective to tap her husband’s office phone in preparation for the commencement of divorce proceedings.

Snoop is a general term that might be used in place of any of the words in this group except the innocent overhear . While snooping has always designated a particularly sneaky, unsavoury way to pry, its conversion from a personal to a electronic technique has given snoop much more sinister connotations than it ever had before.