A polaron deflector was a piece of equipment aboard the USS Defiant. It was being serviced by Chief Miles O'Brien when Lieutenant Thomas Riker came to Deep Space 9 posing as Commander William T. Riker.
O'Brien was working on the deflector's targeting arrays located on the Defiant's bridge when Kira Nerys gave Riker a tour of the Defiant. Riker then commenced with hijacking the ship for the Maquis. (DS9: "Defiant")