Includes one or more segments where the player controls a fixed turret or is unable to move and must fight from where they are (commonly as part of being a passenger in a vehicle). Unlimited ammo is not unusual feature in these.
The first video game about Turret sequence was released on May 21, 2002.
Commonly this has the player controlling the turret or other mounted weapon on a vehicle while the computer drives. This also includes cases where the player is stuck as passenger and must shoot people with the weapons they're carrying.
This differs from simple chance to control a turret by the fact that you must control a turret, there's no choice, no options.