Noun

(en noun)

Close observation of an individual or group; person or persons under suspicion.

Continuous monitoring of disease occurrence for example.

(military, espionage) Systematic observation of places and people by visual, aural, electronic, photographic or other means.

* {{quote-magazine, date=2012-12-14

, author=Simon Jenkins, authorlink=Simon Jenkins , title=We mustn't overreact to North Korea boys' toys , volume=188, issue=2, page=23 , date=2012-12-21 , magazine= citation , passage=The threat of terrorism to the British lies in the overreaction to it of British governments. Each one in turn clicks up the ratchet of surveillance , intrusion and security. Each one diminishes liberty.}}

(legal) In criminal law, an investigation process by which police gather evidence about crimes, or suspected crime, through continued observation of persons or places.