A device that projects the light and force fields making up a hologram, built into holodecks and sickbays and deciding where holographic characters such as Voyager's Doctor can exist.
Holograms can only appear where there are emitters to project them. Holodecks and holosuites are lined with them, and ships with an Emergency Medical Hologram have them in sickbay, which is why Voyager's Doctor is at first confined to that room and the holodeck.
Much of his story in Voyager turns on this limit. Plans to fit emitters elsewhere are discussed in early episodes such as Persistence of Vision. In Future's End he gains a mobile emitter of 29th-century origin and can go anywhere, and Message in a Bottle shows that the newer USS Prometheus has emitters on every deck.
On Deep Space Nine, emitters are set up for Lewis Zimmerman's proposed Long-term Medical Hologram in Doctor Bashir, I Presume, and Quark's holosuites are home to Vic Fontaine.