Sleeper cells are clandestine groups or individuals who remain dormant within a target population for an extended period, sometimes years. They are recruited or self-initiated and blend into the community, appearing ordinary and unassuming. Their purpose is to await activation, at which point they carry out pre-planned operations such as espionage, sabotage, or acts of terrorism. The delayed activation makes them difficult to detect and counter, posing a significant security threat.