A rook (short for "rookie") is a soldier fresh from basic training. Among pilots, a rook is a nugget who has completed flight training and qualifications and is on his or her first tour.
Kara Thrace teases Helo about flying with a rookie, referring to Boomer (Miniseries).
Chief Tyrol refers to several of the deckhands tasked with damage control and fire suppression after the first Cylon attack on Galactica as rooks (Miniseries).
Ensigns Brent Baxton and Joseph Clark are rooks who completed their training on the flight simulators available on Pegasus and are assigned to Galactica afterwards (Scar).