Isolation door

The main engineering isolation door aboard the USS Enterprise-D.

An isolation door is a special reinforced door designed to separate a hazardous area from the rest of a starship or space station. Isolation doors are often employed in Main Engineering, specifically to seal off the warp core and dangerous antimatter reactants in the event of a warp core breach or loss of antimatter containment.

In 2285, an isolation door automatically activated in Engineering aboard the original USS Enterprise following an attack by the USS Reliant. (Star Trek II: The Wrath of Khan)

Strangely, the door cut directly through the horizontal intermix shaft without any obvious cutout or opening, perhaps suggesting the door also interrupted the flow of warp plasma. Additionally, it is unclear what the door was designed to isolate.

While the USS Enterprise-A was under attack by General Chang's Klingon Bird-of-Prey, the isolation door in Engineering rolled down. (Star Trek VI: The Undiscovered Country)

The isolation door aboard the Galaxy class USS Enterprise-D served to separate the warp core and engineer's office from the rest of Engineering. In 2366, the door was brought down after a Borg attack caused a hull breach in the engineering section. (TNG: "The Best of Both Worlds, Part I") It was brought down again after a faulty dilithium chamber hatch ruptured causing an explosion in the dilithium chamber. (TNG: "The Drumhead"). It was used sometime between 2366 and 2368 during an accident in Engineering which claimed the life of Ensign Keller. (TNG: "Violations") In 2371, the door was used in a last-ditch effort to contain the warp core breach which eventually destroyed the stardrive section. (Star Trek: Generations)

In an alternate timeline created when the USS Enterprise-C disappeared from the Battle of Narendra III, the isolation door was used in a vain attempt to contain a coolant leak during a Klingon assault. (TNG: "Yesterday's Enterprise")

The isolation door on the Sovereign class Enterprise-E was seen when two Son'a battle cruisers fired a subspace weapon at the ship. (Star Trek: Insurrection)

Some starships, like the Intrepid class lack isolation doors as their Engineering is extremely open and are separate from the rest of the ship with no open corridors leading to the area. (VOY: "Day of Honor," "Waking Moments")