A pattern enhancer was a type of device that was associated with transporter technology.
The USS Enterprise-D made use of pattern enhancers in 2368 where they were beamed down the surface of an unexplored class M moon of Mab-Bu VI. At the moon's surface was a stranded away team and due to the interference from an approaching storm, a beam out was impossible which prompted Chief Miles O'Brien to transport down with a set of pattern enhancers which were used to safely beam the away team back to the Enterprise. (TNG episode: "Power Play")
The USS Voyager's crew made use of the technology in 2375 to break through a Borg sphere's defenses in order to transport a transwarp coil from the vessel and onto the Voyager. Initial testing first began on the holodeck and was later used to remove the coil from the Borg starship. (VOY episode: "Dark Frontier")
A year later, Voyager's Hazard team deployed pattern enhancers at the Scavengers base in the Forge in order to break through the transport inhibitors that were preventing a beam in or beam out from the base. (VOY video game: Elite Force)