A viewscreen (or main viewer) was an audio/visual device which were used aboard starships, space stations and planetary facilities by various spacefaring organizations, including the Earth Starfleet and Federation Starfleet, the Borg Collective, the Cardassian Union, the Klingon Empire and the Romulan Star Empire.
Generally, a viewscreen was a large screen which was located on the bridge of a starship (or operations center of a facility), and was typically used to display images of the area immediately around the vessel, with the viewscreen allowing views in all directions. Information from a library computer could also be viewed on the main viewer.
One of the main purposes of a viewscreen however, was to allow individuals to see each other face-to-face during ship-to-ship communications. Although the ship had to be in visual range in order to receive a visual communication. (TOS movie, novelization & comic adaptation: The Final Frontier; TNG episode: "Ship in a Bottle")
- Contrary to later sources, Life Form Nonexistent describes the USS Enterprise's viewscreen as a large window, concealed behind a retractable panel when not in use, similar to the viewscreen windows seen in the 2009 movie Star Trek.