Executive officer
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The second in command aboard a Battlestar, reporting directly to the Commanding Officer.
Aboard the Battlestar Galactica this position is held by Saul Tigh. He is responsible for the day-to-day operation of the ship, and takes command in William Adama's absence.
Note
Executive Officer is a military term in this context, used in several branches of the armed forces worldwide (naval, marines, air force). Certain forces, such as the British Army use the term "2i/c" (signifying second in command) in preference to XO.