Definition - What does Surveillance mean?
Surveillance is when members of law enforcement carefully observe and monitor certain activities of a person, typically a suspect in a crime, in order to gather information. This information can be used as evidence in a court of law. However, the surveillance must yield useful material if it is to be used as evidence in the case.
Justipedia explains Surveillance
Surveillance can also be used to prevent crimes. For example, police officers may conduct surveillance at a particular location to see if criminal activity is occurring. If it is, then they might arrest the people involved.
Occasionally, law enforcement may conduct surveillance on a suspected criminal's phone line to monitor for information or evidence. However, a warrant is usually necessary to conduct this type of surveillance. If the warrant is granted, law enforcement can essentially listen in on the conversations that the suspect has on the phone.