The communicator is a hand-held device used by Starfleet crews to talk with their ship and each other, known in the 23rd century for its flip-open design.
Communicators let landing parties stay in contact with their starship and provided a signal the transporter could use to locate them. In the original era they were compact units with a hinged grille that opened to activate the device, and losing one could leave a crew member stranded.
Earlier models were already in use aboard Enterprise NX-01 in the 22nd century, and the flip-open design continued through the 23rd. By the 24th century the hand-held unit had largely been replaced by the communicator badge worn on the uniform.
Communicators also caused trouble when they went astray. When Kirk's team visited Sigma Iotia II in 2268, a communicator was accidentally left behind, raising concerns about what the imitative Iotians might learn from it.