An E-band transmitter is a device to transmit signals in the E-band frequency of the electromagnetic spectrum.
Klingon ambassador Kell, secretly working for the Romulans, used an E-band transmitter to covertly send instructions to a brainwashed Geordi La Forge. He couldn't get too far away from La Forge in order for this to work as the signals had to be transmitted in close proximity to La Forge's VISOR. (TNG: "The Mind's Eye")