Parc fermé is French for "closed park". In Formula 1 it refers to a set of rules, in force from the start of qualifying through to the race, during which teams are heavily restricted from modifying their cars. The aim is to stop teams from running a low-downforce, qualifying-only setup and then transforming the car for the race.
Once a car enters parc fermé conditions, the setup is essentially frozen. Teams may carry out routine work — refuelling for sessions where permitted, changing tyres and brake pads, topping up fluids, and so on — but they cannot make meaningful changes to the suspension, aerodynamic setup, or other performance-defining parts without permission.
Limited adjustments are allowed, such as front-wing flap angle, and teams can ask stewards for approval to fix genuine damage or safety issues. If a team makes a change that is not permitted, the driver typically has to start the race from the pit lane rather than their qualified grid position.
Parc fermé also describes the physical area where cars are held and inspected after qualifying and the race, where scrutineers check them for technical compliance. The principle is the same in both senses: the car is locked to a fixed specification so the competition stays fair.