Aim dead zone

This can be combined with independent aim in some rare cases, usually involves a locomotive part (e.g. tank tracks), the turret which provides the basis for independent aim tag, while the cannon itself may allow some minimal nudging itself without affecting the turret mount's heading (like most towed cannons).

Usually this is seen in first person games, but not necessarily limited to them, where the player can aim around the middle of the screen without changing the view direction at all.

Good older examples of this are: System Shock [1], CyClones and MechWarrior.

See also: independent aiming (a more complete variant of this), floating aim (similar, but does not prevent view direction changes within the free aim zone)