The USS Monitor (NCC-61826) was a Nebula-class Federation starship that was in service during the latter half of the 24th century.
In 2366, the Monitor was dispatched to the Romulan Neutral Zone in order to counter a possible Romulan sneak attack originating from a supposed cloaked Romulan base on Nelvana III. (TNG: "The Defector")
In early 2367, the Monitor patrolled the Romulan Neutral Zone, along with the USS Berlin. (TNG: "Brothers")
Appedendices
Background
The class and registry were derived from an Okudagram displayed in the observation lounge starting around TNG Season 4. This information appeared on the Starship Mission Status screen.
Named after the innovative, steam-powered ironclad which fought for the Union in 1862, during the American Civil War. President Abraham Lincoln toured the vessel after its battle with the CSS Virginia (the former USS Merrimack).
Apocrypha
According to the non-canonical Star Fleet Technical Manual, there was a Constitution-class USS Monitor in service in the 23rd century, with the registry NCC-1713.
In William Shatner's book "The Return", the USS Monitor is a Defiant-class vessel that is temporarily renamed Enterprise while mounting a direct offensive against the Borg/Romulan alliance at the Borg homeworld.
In the video game Star Trek: Away Team, the USS Monitor is a Galaxy-class starship, that is being assimilated by the Borg during one of the missions.
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