Bulkhead

As per standard Union Navy terms and idiom permited for official use the word Bulkead refers to any exterior wall of a ship (or space station) or any upright partition and part of the structural rigidity of the vessel.

The command "Bulk head Lock"  or  "Lock Down" refers to the closing and locking of all Bulk head doors, hatchways or any closeable hole or structure. To prevent the spread of fire, loss of atmosphere or the containment of intruders or mutineers.

bulk·head (blkhd) n.'''1. a. One of the upright partitions dividing a ship into compartments and serving to add structural rigidity and to prevent the spread of leakage or fire.b. A partition or wall serving a similar purpose in a vehicle, such as an aircraft or spacecraft.2. A wall or an embankment, as in a mine or along a waterfront, that acts as a protective barrier.3. 'Chiefly New England'' A horizontal or sloping structure providing access to a cellar stairway.