What exactly is trim? Although I have a rough idea of what it is...

In my mind from growing up watching hundreds of hours of Seconds from Disaster / Air Crash Investigation, trimming an aircraft is turning a dial that changes the airplanes "forward" direction. So maybe it changes the control surfaces so that the plane is flying straight forward even though the rudder for example is at some deflection angle that isn't 0.

However if I was asked to define trim I couldn't exactly say what it is, what situations I would use it for or how it's achieved.
If the aircraft is trimmed, the flight controls are in a state where no force needs to be exerted in order to continue straight and level flight. Aerodynamic and gravitational moments about all three axes are nulled out.

Changing the trim of the aircraft in pitch could happen in the following ways:


a shift in centre of gravity, for instance by shifting fuel from centre tanks to tail tanks, or payload shifting position;
a change in thrust of an engine that is located above or below the centre of gravity;
a change in horizontal tail angle or trim tab;
a deflection of the elevator.


If the aircraft was in straight and level flight and the trim changes because of a change in any of the above, it can be compensated for by the other factors. Elevator deflection is fastest but requires a pilot force which is uncomfortable for longer periods, that is why the forces can be trimmed out to make the new stick position the neutral force one.