A mobile emitter is a piece of technology from the twenty-ninth century in the Star Trek continuum. It is used by holographic characters to travel outside of holodecks and other specially-equipped areas. Voyager's EMH has one, as does the Bashir Hologram.
In the PPC, Agent Cornelius, an artificial intelligence in the DIAU, uses a mobile emitter combined with a D.O.R.K.S. to project a holographic form for use during missions. Unfortunately, the combination of technologies is unstable, so the form can only be maintained for short periods of time.