In the contex of military context, surveillance is the process of observation of aerospace, surface, or subsurface areas, places, persons, or things. This process may use technical sensors, native human abilities or both. Surveillance differs from reconnaissance in that surveillance is a continuing process, where reconnaissance is carried out by specific missions.
Another usage of surveillance, in law enforcement or non-military security, is the monitoring of people and their communications and activities.
The term sousveillance (in French, "sur" means over and "sous" under) is sometimes used for the reverse, for example citizens filming police activity.
Contrasting military surveillance and reconnaissance
For example, a surveillance system might use a ground surveillance radar to detect, on a 24-hour basis, activity along a border. Reconnaissance of the same border would involve a ground patrol or overflight along it.
Intelligence, in the context of intelligence, surveillance, and reconnaissance is the synthesis and analysis of information produced by surveillance and reconnaissance.