A transporter scrambler (also called a transport scrambler or pattern scrambler) was a device designed to discourage transporter use in a certain area by interfering with rematerialization. Transport scramblers could be programmed to recognize particular signals and only interfere with enemy transporter systems.
During the Klingon invasion of Ajilon Prime in early 2373, they used transport scramblers in ground battles to prevent the enemy from deploying troops effectively. (DS9: "Nor the Battle to the Strong")
A transporter scrambler was part of the security system installed aboard Deep Space 9. In 2373, Lupaza found a way to re-tune the scrambler and beam into Kira Nerys' quarters. (DS9: "The Darkness and the Light")
The Cardassians routinely booby trapped their abandoned facilities, such as Empok Nor, with pattern scramblers rigged to activate if they detect a non-Cardassian transporter signal. Miles O'Brien remarked in 2373 that these devices could be "messy". (DS9: "Empok Nor")