Definition from Nolo’s Plain-English Law Dictionary
The act of observing persons or groups either with notice or their knowledge (overt surveillance) or without their knowledge (covert surveillance). Intrusive surveillance by private citizens may give rise to claims of invasion of privacy. Police officers, as long as they are in a place they have a right to be, can use virtually any type of surveillance device to observe property. Police cannot use specialized heat-scanning surveillance devices to obtain evidence of criminal activity inside a home. Law enforcement officials acquired additional surveillance capability following enactment of The Patriot Act.