A clandestine operation is an intelligence or military operation carried out in such a way that the operation goes unnoticed by the general population or specific 'enemy' forces.
The United States Department of Defense Dictionary of Military and Associated Terms (Joint Publication JP1-02, dated 5 January 2007) defines "clandestine operation" as "An operation sponsored or conducted by governmental departments or agencies in such a way as to assure secrecy or concealment. A clandestine operation differs from a covert operation in that emphasis is placed on concealment of the operation rather than on concealment of the identity of the sponsor. In special operations, an activity may be both covert and clandestine and may focus equally on operational considerations and intelligence-related activities." (JP 3-05.1).
The bulk of clandestine operations are related to the gathering of intelligence, typically by both people (Clandestine HUMINT) and by hidden sensors. Placement of underwater or land-based communications cable taps, cameras, microphones, traffic sensors, monitors such as sniffers, and similar systems require that the mission go undetected and unsuspected. Clandestine sensors may also be on unmanned underwater vehicles, reconnaissance (spy) satellites (such as Misty), low-observability unmanned aerial vehicles (UAV), or unmanned detectors (as in Operation Igloo White and its successors), or hand-placed by clandestine human operations.