An electronic component described by Spock as the basis of every important piece of Starfleet equipment, from the communicator to the transporter and the ship's engines.
The transtator is mentioned in the closing scene of the episode A Piece of the Action. Having forced the rival gang bosses of Sigma Iotia II into a single government, Kirk, Spock and McCoy return to the Enterprise satisfied with their work.
McCoy then admits that he has left his communicator behind in the office of Bela Oxmyx. Spock points out the danger: the device contains a transtator, the element on which almost all Federation technology depends.
The Iotians are brilliant imitators who built an entire society from one book left by an earlier ship. Given a working example, Kirk expects them to take it apart, understand it, and one day arrive wanting a share of the Federation's action.