A cell phone jammer is a device that blocks transmission or reception of signals, usually by creating some form of interference at the same frequency ranges that cell phones use. As a result, a cell phone user will either lose the signal or experience a significant loss of signal quality.
Cell phone jammers have both benign and malicious uses. Police and the military often use them to limit or disrupt communications during hostage situations, bomb threats or when military action is underway. Portable personal jammers are available to enable their owners to stop others in their immediate vicinity (up to 60-80 feet away) from using cell phones.
Similar equipment is manufactured to block signals in environments where cell phone activity may not be desirable, such as theaters, churches and operating rooms.
Cell phone jammers can not legally be sold or used within the United States.