By your command
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"By your command" is arguably the most popular phrase of the Battlestar Galactica saga.
Original Series usage
The phrase is frequently said by the robotic soldiers of the Cylon Empire in the Original Series in a neutral metallic voice as the affirmative response to an order from another Cylon Centurion, the Imperious Leader, or other authority (including Baltar). The phrase is iconic of the Cylons' nature as a robotic race, becoming something of a 'catchphrase' of the series.
Re-imagined Series/Caprica usage
In the Re-imagined Series, the phrase has been said twice by copies of Number Six, once to a copy of Sharon Valerii in response to finding the escaped Fleet of refugee survivors of the genocidal Cylon attack on the Colonies (Miniseries), and again (barely audible) in the episode "Colonial Day."
Since these Cylons appear to run their fleet by consensus, the use of "By your command" may be less of a command acknowledgment. The use of the phrase may be more analogous to the Colonial affirmation "So say we all."
The catchphrase is also used in "Razor" by old model Cylons attacking Pegasus. These old model Cylons belong to a group of Centurions known as "Guardians" who escaped being scrapped when the model was phased out.
Its origins are revealed in the Caprica pilot, when the U-87 Cyber Combat Unit, the prototype of the original Cylon Centurions in the Twelve Colonies, completes its first successful live-fire exercise and reports to its human observers, "Program completed by your command." As U-87s enter mass production and are put into service, "By your command" is used regularly (Caprica).
The phrase appears to be rare among non-Cylon robots in the Twelve Colonies. When asked about whether he could use the response himself, Serge stated that it was an available option, but that it sounded "a little stiff."