What is Surveillance?

Legal Definition
Surveillance (/sərˈv.əns/ or /sərˈvləns/) is the monitoring of the behavior, activities, or other changing information, usually of people for the purpose of influencing, managing, directing, or protecting them. This can include observation from a distance by means of electronic equipment (such as CCTV cameras), or interception of electronically transmitted information (such as Internet traffic or phone calls); and it can include simple, no- or relatively low-technology methods such as human intelligence agents and postal interception. The word surveillance comes from a French phrase for "watching over" (sur means "from above" and veiller means "to watch"), and is in contrast to more recent developments such as sousveillance.

Surveillance is used by governments for intelligence gathering, the prevention of crime, the protection of a process, person, group or object, or the investigation of crime. It is also used by criminal organizations to plan and commit crimes such as robbery and kidnapping, by businesses to gather intelligence, and by private investigators.

Surveillance is often a violation of privacy, and is opposed by various civil liberties groups and activists. Liberal democracies have laws which restrict domestic government and private use of surveillance, usually limiting it to circumstances where public safety is at risk. Authoritarian government seldom have any domestic restrictions, and international espionage is common among all types of countries.
-- Wikipedia
Legal Definition
Observation and collection of data to provide evidence for a purpose.