Meaning: Principal officer in command.
A real-world communications term signifying that the officer-in-charge (e.g. the commander of a military vessel, task force or unit) is speaking, or is required on-air.
Used to distinguish the commanding officer from other personnel who may use the communications channel, without recourse to code names or other means of identification. Frequently used when important / personal / restricted information needs to be communicated directly to the officer-in-charge.
William Adama is known as Galactica's Actual.