Arch
An arch was a structure in 24th century Starfleet holodecks which surrounded their entrance doors. It had computer control panels and interfaces when participants wished to modify the current program, communicate throughout the ship, or access most general computer functions. (; )

When a holographic program was running, the arch was typically hidden holographically. It could be made to materialize by voice command. The arch was then visible to participants, and even though the holographic characters generally did not perceive the arch, there were exceptions. (; )

A related voice command, "Exit", was used to remove the holographic environment around the arch and holodeck door when participants wished to leave the holodeck without closing the program. (, et al.)

Holographic technology