Close observation of an individual or group; person or persons under suspicion.
Systematic observation of places and people by visual, aural, electronic, photographic or other means.
Continuous monitoring of disease occurrence for example.
In criminal law, an investigation process by which police gather evidence about crimes, or suspected crime, through continued observation of persons or places.
An indication that someone or something is in danger or at risk
A sign that indicates a danger, for example to vehicles on a road or of hazardous chemicals.