Gimbal, thrust vectoring or thrust vector control (TVC) is the ability of engines to alter the direction of thrust by pivoting. Most liquid fuel engines and all jet engines [outdated] support thrust vectoring. This feature increases the turn rate of a craft.
The Stability Augmentation System uses this system to stabilize the vessel, but sometimes the effect is so large that it tears the craft apart. To reduce this hazard it is possible to lock gimbal for each engine separately, which can also be done with action groups.
Prior to 0.24 thrust vectoring was only used for pitch and yawing. With 0.24 engines not placed along the roll axis now participate controlling roll. Also engines with multiple nozzles not along the roll axis of the engine participate, even if the engine is placed along the roll axis, because the nozzles itself are not. This means that every nozzle can be gimballed separately.