A hidden camera or spy camera or security camera is a still or video camera used to record people without their knowledge. The camera is “hidden” because it is either not visible to the subject being filmed, or is disguised as another object. Hidden cameras can be built into commonly used objects such as television sets, smoke detectors, clock radios, motion detectors, ball caps, plants, and mobile phones. Hidden cameras may be used for household surveillance and may also be used commercially or industrially as security cameras. The proliferation and lower costs of video recording devices has led to an increase in the use of hidden cameras for legitimate surveillance need, as well as for entertainment and other purposes.
The use of hidden cameras raises personal privacy issues and there may be legal aspects to consider, depending on the jurisdiction in which use takes place. A hidden camera can be contrasted with CCTV, which is visible and which sometimes is accompanied with a warning notice of its presence.
Source: wikipedia & youtube