A bulkhead is a wall-like partition in a starship used to aid structural integrity.
Bulkheads are numbered and divide a ship into separate parts to help keeping its rigidity. Bulkheads in Starfleet vessels built in the 24th century, specifically of the Galaxy-class and Intrepid-class, were composed of tritanium alloy and could be reinforced with shields to enhance the ship's overall structural integrity. (TNG: "Where Silence Has Lease"; VOY: "Repentance")
In case of structural failure, a bulkhead can be beamed away and temporarily replaced by a force field. (TNG: "Schisms")
In 2369 the bulkheads aboard Deep Space 9 were broken by several hull breaches caused by graviton pulses of the embryonic lifeform. (DS9: "Q-Less")
While searching the debris of the destroyed Vor'cha-class attack cruiser IKS Toh'Kaht, Dax and O'Brien found pieces of the secondary hull and the bulkheads in the debris. (DS9: "Dramatis Personae")
Jadzia Dax claimed she could hear Kor snoring through the bulkheads of the USS Rio Grande in 2372. (DS9: "The Sword of Kahless")
Elim Garak had "a sudden and somewhat violent encounter with a bulkhead" when the USS Defiant was attacked by the Dominion early in the war. (DS9: "A Time to Stand")