An Arch is the computer control panel on the holodeck of 24th century starships. When a program is running in the holodeck, the arch could be made to materialize by voice command when participants wish to modify the current program, communicate throughout the ship, or access most general computer functions. (e.g. Star Trek: Generations)
A related voice command, "Exit", is used to cause the exit door to materialize when participants wished to leave the holodeck. Although the arch is visible to participants, the holographic characters generally do not perceive the arch, although there have been exceptions. (TNG: "Elementary, Dear Data")