A coded transponder frequency, or transponder code, was a system of identification used by starships in the 24th century. Once deciphered, a vessel's transponder code would allow other ships in the vicinity to pinpoint its precise location.
In 2367, Gul Macet asked Captain Picard to transmit the transponder code of the USS Phoenix to one of his warships. Picard acquiesced, as it was the only way to prevent the renegade Phoenix from destroying a Cardassian supply ship. Macet was surprised to learn that Starfleet had already found a way to read Cardassian transponder codes. (TNG: "The Wounded")