Wiretapping
In short
Wiretapping is the secret monitoring of telephone or internet-based conversations by a third party. It can be done passively to listen or actively to change the communication.
In plain words
Someone listens to your calls or reads your digital messages without you knowing. This can be done by criminals to steal secrets or by the police to catch criminals.
A simple example
A hacker installs software on a smartphone to record every private phone call made by the owner. Alternatively, a detective uses a court order to listen to a suspect's phone line.
Why it matters
It is a major issue for personal privacy and national security. It forces technology companies to create better encryption to protect users from being spied on.
Easy to mix up
Do not confuse it with eavesdropping, which is generally just listening in on a conversation in a public place.