Trim Systems are considered to be a "secondary" flight control system.
By definition, to "trim" an aircraft is to adjust the aerodynamic forces on the control surfaces so that the aircraft maintains the set attitude without any control input. While all axes of rotation are affected by aerodynamic forces, not all aircraft types are capable of being trimmed in all three axes. Virtually all aircraft designs incorporate some form of pitch axis trim and most have provisions of some description for trimming in the yaw axis. Roll axis trim exists on many aircraft but it is the least frequently encountered installation of the three. There are several different types of trim systems in use and more than one type may be found on a given aircraft.
The most commonly used trim system is the trim tab which can be installed in both fixed or flight adjustable configurations. Adjustable springs, anti-servo tabs on a stabilator equipped aircraft and a trimmable horizontal stabiliser are other types of trim systems.