Combat Air Patrol
From Battlestar Wiki, the free, open content Battlestar Galactica encyclopedia and episode guide
CAPs typically consist of two Vipers (lead and wingman) flying a designated pattern in and around the Fleet, providing an immediate spaceborne response to incoming threats. More than one CAP may be flown at a time, with Vipers positioned at different distances and "orbits" around the Fleet. CAP flights can be reinforced by a battlestar's alert fighters (TRS: "33").
CAPs sometimes include Raptors in a support role.
Despite the 40 Vipers at her disposal, Kara Thrace finds mounting full CAPs difficult in the episode "Sine Qua Non". It may be that on high alert Galactica mounts multiple simultaneous CAPS, along with the 2 Vipers sometimes seen on screen escorting Colonial One. Flying continuous CAP shifts with alert fighters on standby could explain this strain on pilots and planes.
Later in the series, it becomes even more difficult to keep up a full CAP. Casualties, the mutiny, and a rebel Cylon baseship joing the Fleet result in a joint CAP being flown including both Colonial Vipers and Cylon Heavy Raiders (TRS: "Someone to Watch Over Me").