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Memory Alpha
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(covers information from several alternate timelines)

The second officer was an officer who was third in command of a starship and who assumed command of the ship when both the captain and first officer were unavailable.

It was standard procedure for a Starfleet second officer to accompany the away team. (TNG: "Time's Arrow")

Ferengi code was to offer the life of their second officers when surrendering. When Lieutenant Commander Data, second officer of the USS Enterprise, learned of this code in 2364, he stated to Geordi La Forge, "Fortunately, Starfleet has no such rules involving our second officers." (TNG: "The Last Outpost")

During a second encounter with the Ferengi that year, Data was introduced to Bok by Captain Jean-Luc Picard as his "second-in-command", after Bok's own second officer was Rata was introduced. (TNG: "The Battle")

As noted above Data was referred to by Picard as his "second-in-command" (rather than "third-in-command") during the same introduction of Riker as his "first officer", both are the same title.
In similar regards, Spock referred to himself as "second officer" in TOS: "The Enemy Within", rather than "second-in-command" or "first officer".

Still later that year, Bjorn Bensen, the chief engineer of Velara Base, was intrigued by Data, asking if he was an android. Tasha Yar, answered that he was, and added that additional he was "third-in-command of the Enterprise." (TNG: "Home Soil")

On several occasions thereafter, he was introduced as the ship's second officer. (TNG: "First Contact", "Time's Arrow", "Starship Mine")

On Klingon warships, the first officer was called upon to ensure their incapacitated captain died an honorable death. In 2365, Worf explained to William T. Riker that should he also become unable to perform while serving as an exchange officer, his second officer would kill him. (TNG: "A Matter Of Honor", "Shades of Gray")

In a follow up to this exchange, Kargan also introduced Klag as Riker's second officer, rather than as the ship's second officer.

When the Satarran agent using the alias of Kieran MacDuff commandeered the Enterprise in 2368, he altered the computer records so that the ship's rightful first officer, Commander Riker, was listed as the second officer. (TNG: "Conundrum")

List of second officers

See also

External link