Final answer:
The horizontal or inclined door over a stairway is called a Bulkhead, often leading to a basement and designed to prevent water entry.
Step-by-step explanation:
The term for a horizontal or inclined door over a stairway is Bulkhead. This type of door typically serves as an external entryway to a basement and can be found at ground level, leading down to below-ground space. Bulkheads provide access while also preventing water and debris from entering the stairwell, hence their sometimes slanted or inclined design, which assists in shedding water away from the entrance.