Covariant pulse
From Memory Alpha, the free Star Trek reference
A covariant pulse is a type of energy that can be transmitted over interstellar distances.
In 2373, after his salvage team was trapped aboard Empok Nor by soldiers of the Cardassian Third Order, Miles O'Brien devised a plan to contact Deep Space 9 by modifying the field coils of the station's deflector array to emit covariant pulses, communicating in a way analogous to a telegraph. However, the plan could not be enacted until they neutralized Elim Garak, who had been driven by a psychotropic drug into a murderous delusional state. (DS9: "Empok Nor")
